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.
I'm not a big fan of using those because they don't have reflectors built into them. The reflector that sits behind the bulb on a reptile specific or grow light fixtures helps direct more of the UVB downward from the fixture, increasing its effective output. This is especially important with the weaker T8 fixtures/bulbs.
Looking at the cost of a T8 vs T5, a T8 might not be the most cost effective solution. A T5 bulb is stronger and requires replacement less frequently (roughly every 6mo for a T8 vs roughly 1yr+ for a T5 bulb).
It also depends on the distance you need to reach in order to provide adequate UVB. The optimal distance for a T5 fixture with a ReptiSun 5.0 bulb is around 6-8'' - about the same as a T8 - while the T5 with a ReptiSun 10.0 bulb has an optimal distance of around 12-14'' or so.