If you can't find the right size, you may try my approach:
I have incandecent light for a 10 gallon, but I replaced the bulbs with flurescent ones from wal-mart. They are actually florescent bulbs that are tubes in a spiral, and fits the socket space.
I got 2 100W bulbs that are bright enough that I use for my 40 gallon tank, and the bulbs cost me about 16 dollars.
There are 40W bulbs, and they should be cheaper as well.