Zilla Slimline is probably the most commonly available 18'' T8 fixture in the US. It's probably a good idea to replace the bulb with a T8 ReptiSun 10.0 though. Those fixtures come with a piece of clear plastic over the bulb that needs to be removed before using it, otherwise the plastic will intercept all of the UVB. The optimal distance for these fixture/bulb combination is about 6-8'' above the main basking area - so they're best suited for small enclosures.
Moved by moderator from Off Topic to Enclosures.