A great place to search for information is the manufacturer itself.
At "http://www.boeing.com/commercial/airports/plan_manuals.html" select your aircraft.
Then select "2.0 Airplane Description" and you get all the Information you need.
For Example:
B737-800
Usable Fuel = 26.022 Liter
B737 BBJ 2 (Based on -800)
Usable Fuel = 27.992 Liter (with 1 aditional Aux Tank)
Usable Fuel = 29.665 Liter (with 2 aditional Aux Tanks)
Usable Fuel = 31.645 Liter (with 3 aditional Aux Tanks)
Usable Fuel = 33.609 Liter (with 4 aditional Aux Tanks)
Usable Fuel = 35.576 Liter (with 5 aditional Aux Tanks)
Usable Fuel = 37.538 Liter (with 6 aditional Aux Tanks)
Usable Fuel = 39.530 Liter (with 7 aditional Aux Tanks)