Maybe 15W is the highest aquarium bulb you can find, but you can use any cool white (6500k) CFL.
But then again, 2 15W fixtures would be enough light for most plants, without the need for CO2 and heavy ferts.
(I have 20W over a 10G, and it's almost too much light)
30W for a 20G would more than likely be plenty- if not, just get 2 18W cool white CFLs.
36W would definately be enough... but would likely be on the bright side.
Remember, the WPG rule was made to measure T5 (normal output) Flourescent tubes-
CFLs are nearly as efficient as t8.
If you want 18's that are guaranteed to fit, I found some.
*18 Watt 6400 K Daylight - $8.99
Don't even think about getting the SHO on that page. You'd blind your fishes.