A while ago I created a tab on my own version of my spreadsheet that contains all lands, and I have included 5 columns which I have populated with the type of mana they produce. A simple filter can then provide me with all lands that can produce the required colours.
So for BUW, I find just the following:
Arcane Sanctum http://magiccards.info/ala/en/220.html
Arcane Sanctum enters the battlefield tapped.
{T}: Add {W}, {U}, or {B} to your mana pool.
Dromar's Cavern http://magiccards.info/ps/en/138.html
When Dromar's Cavern enters the battlefield, sacrifice it unless you return a non-Lair land you control to its owner's hand.
{T}: Add {W}, {U}, or {B} to your mana pool.
As Jeff has pointed out there are plenty of "Fetch Lands"
Esper Panorama http://magiccards.info/ala/en/223.html
{T}: Add {1} to your mana pool.
{1}, {T}, Sacrifice Esper Panorama: Search your library for a basic Plains, Island, or Swamp card and put it onto the battlefield tapped. Then shuffle your library.
Terramorphic Expanse http://magiccards.info/cmd/en/291.html
{T}, Sacrifice Terramorphic Expanse: Search your library for a basic land card and put it onto the battlefield tapped. Then shuffle your library.
Evolving Wilds http://magiccards.info/roe/en/228.html
{T}, Sacrifice Evolving Wilds: Search your library for a basic land card and put it onto the battlefield tapped. Then shuffle your library.
Thawing Glaciers http://magiccards.info/ai/en/189.html
Thawing Glaciers enters the battlefield tapped.
{1}, {T}: Search your library for a basic land card, put that card onto the battlefield tapped, then shuffle your library. Return Thawing Glaciers to its owner's hand at the beginning of the next cleanup step.
Terminal Moraine http://magiccards.info/ps/en/142.html
{T}: Add {1} to your mana pool.
{2}, {T}, Sacrifice Terminal Moraine: Search your library for a basic land card and put that card onto the battlefield tapped. Then shuffle your library.
And there are then a large number of lands that can produce any colour, some with more restrictions than others:
Ancient Ziggurat http://magiccards.info/cfx/en/141.html
{T}: Add one mana of any color to your mana pool. Spend this mana only to cast creature spells.
Cavern of Souls http://magiccards.info/avr/en/226.html
As Cavern of Souls enters the battlefield, choose a creature type.
{T}: Add {1} to your mana pool.
{T}: Add one mana of any color to your mana pool. Spend this mana only to cast a creature spell of the chosen type, and that spell can't be countered.
Unstable Frontier http://magiccards.info/cfx/en/145.html
{T}: Add {1} to your mana pool.
{T}: Target land you control becomes the basic land type of your choice until end of turn.
Shimmering Grotto http://magiccards.info/lw/en/273.html
{T}: Add {1} to your mana pool.
{1}, {T}: Add one mana of any color to your mana pool.
School of the Unseen http://magiccards.info/ai/en/186.html
{T}: Add {1} to your mana pool.
{2}, {T}: Add one mana of any color to your mana pool.
Tendo Ice Bridge http://magiccards.info/bok/en/165.html
Tendo Ice Bridge enters the battlefield with a charge counter on it.
{T}: Add {1} to your mana pool.
{T}, Remove a charge counter from Tendo Ice Bridge: Add one mana of any color to your mana pool.
Mirrodin's Core http://magiccards.info/ds/en/165.html
{T}: Add {1} to your mana pool.
{T}: Put a charge counter on Mirrodin's Core.
{T}, Remove a charge counter from Mirrodin's Core: Add one mana of any color to your mana pool.
Pillar of the Paruns http://magiccards.info/di/en/176.html
{T}: Add one mana of any color to your mana pool. Spend this mana only to cast multicolored spells.
Rupture Spire http://magiccards.info/ddh/en/75.html
Rupture Spire enters the battlefield tapped.
When Rupture Spire enters the battlefield, sacrifice it unless you pay {1}.
{T}: Add one mana of any color to your mana pool.
Forbidden Orchard http://magiccards.info/v12/en/6.html
{T}: Add one mana of any color to your mana pool.
Whenever you tap Forbidden Orchard for mana, put a 1/1 colorless Spirit creature token onto the battlefield under target opponent's control.
Vesuva http://magiccards.info/v12/en/14.html
You may have Vesuva enter the battlefield tapped as a copy of any land on the battlefield.
Archaeological Dig http://magiccards.info/in/en/320.html
{T}: Add {1} to your mana pool.
{T}, Sacrifice Archaeological Dig: Add one mana of any color to your mana pool.
Saprazzan Skerry http://magiccards.info/mm/en/328.html
Saprazzan Skerry enters the battlefield tapped with two depletion counters on it.
{T}, Remove a depletion counter from Saprazzan Skerry: Add {U}{U} to your mana pool. If there are no depletion counters on Saprazzan Skerry, sacrifice it.
Glimmervoid http://magiccards.info/mi/en/281.html
At the beginning of the end step, if you control no artifacts, sacrifice Glimmervoid.
{T}: Add one mana of any color to your mana pool.
Primal Beyond http://magiccards.info/mt/en/149.html
As Primal Beyond enters the battlefield, you may reveal an Elemental card from your hand. If you don't, Primal Beyond enters the battlefield tapped.
{T}: Add {1} to your mana pool.
{T}: Add one mana of any color to your mana pool. Spend this mana only to cast Elemental spells or activate abilities of Elementals.
Tarnished Citadel http://magiccards.info/od/en/329.html
{T}: Add {1} to your mana pool.
{T}: Add one mana of any color to your mana pool. Tarnished Citadel deals 3 damage to you.
Grand Coliseum http://magiccards.info/on/en/319.html
Grand Coliseum enters the battlefield tapped.
{T}: Add {1} to your mana pool.
{T}: Add one mana of any color to your mana pool. Grand Coliseum deals 1 damage to you.
Reflecting Pool http://magiccards.info/tp/en/328.html
{T}: Add to your mana pool one mana of any type that a land you control could produce.
Gemstone Mine http://magiccards.info/tsts/en/119.html
Gemstone Mine enters the battlefield with three mining counters on it.
{T}, Remove a mining counter from Gemstone Mine: Add one mana of any color to your mana pool. If there are no mining counters on Gemstone Mine, sacrifice it.
Undiscovered Paradise http://magiccards.info/vi/en/167.html
{T}: Add one mana of any color to your mana pool. During your next untap step, as you untap your permanents, return Undiscovered Paradise to its owner's hand.
Lotus Vale http://magiccards.info/wl/en/165.html
If Lotus Vale would enter the battlefield, sacrifice two untapped lands instead. If you do, put Lotus Vale onto the battlefield. If you don't, put it into its owner's graveyard.
{T}: Add three mana of any one color to your mana pool.
Then there are more unusual ways of getting multi-coloured mana:
Springjack Pasture http://magiccards.info/eve/en/179.html
{T}: Add {1} to your mana pool.
{4}, {T}: Put a 0/1 white Goat creature token onto the battlefield.
{T}, Sacrifice X Goats: Add X mana of any one color to your mana pool. You gain X life.
Rainbow Vale http://magiccards.info/med/en/179.html
{T}: Add one mana of any color to your mana pool. An opponent gains control of Rainbow Vale at the beginning of the next end step.
Crystal Quarry http://magiccards.info/od/en/318.html
{T}: Add {1} to your mana pool.
{5}, {T}: Add {W}{U}{B}{R}{G} to your mana pool.
Exotic Orchard http://magiccards.info/pc2/en/117.html
{T}: Add to your mana pool one mana of any color that a land an opponent controls could produce.
Forsaken City http://magiccards.info/ps/en/139.html
Forsaken City doesn't untap during your untap step.
At the beginning of your upkeep, you may exile a card from your hand. If you do, untap Forsaken City.
{T}: Add one mana of any color to your mana pool.
Meteor Crater http://magiccards.info/ps/en/140.html
{T}: Choose a color of a permanent you control. Add one mana of that color to your mana pool.
Rhystic Cave http://magiccards.info/pr/en/142.html
{T}: Choose a color. Add one mana of that color to your mana pool unless any player pays {1}. Activate this ability only any time you could cast an instant.
Thran Quarry http://magiccards.info/sus/en/1.html
At the beginning of the end step, if you control no creatures, sacrifice Thran Quarry.
{T}: Add one mana of any color to your mana pool.
