There are two types of fixtures. One with an 18" T8 fluorescent, or one with two sockets for incandescent bulbs. Both will work just fine.
For the one with two sockets, just buy two 10W CFLs at the 6500K color temperature. You should be able to find them in hardware stores and they will cost about $10. This will be on the high side of moderate lighting so you'll want floating plants unless you are doing CO2 injection.
In both cases you'll have to at least buy the fixture, and possibly a hood if the one you have will not work with a standard light fixture.
It's possible to find an 18" fluorescent shop light that would work as well and be cheaper, but you must find one you can use without the bulb touching the hood. Do not get a dual T8 fixture as that would be too much light.
You can also get a couple clamp on lights to use CFLs in. Just don't clamp them to the tank.
In all cases, you need a hood to not only prevent the fixture from falling into the water, but to keep the humidity off the light fixtures. Hoods can be glass (best) or plexiglass cut to size (bends easily, get a thick piece).