I have a fluorescent UVB tube that has no manufactured fixture. It's a Zoomed lamp, so I don't want to pay their high price for a fixture they have that fits this tube. I've already found the ballast by calling lighting supply stores. Then all I need are the end caps, which for this T8 tube are very common. Then I assemble all the stuff at home.
A quirk I have is to mount the ballast outside the habitat, because it's a heat source. I run the four wires down into the habitat to the lamp, bundle them with bits of tape, and go to each end-cap with two wires. I like to have full control of the heat that is generated within the habitat. I don't want an overly warm ballast hanging around in there.
The code for the fixture is written near the end of the fluorescent tube. The only thing you need to do is to specify how many tubes you will be placing in one fixture.
Call lighting supply stores until you find one that is willing to help you.
For instance, I have a tube with no fixture. The specs are near one end of the tube, as follows:
1)Inches. 15"/381 nm (The nm is the wavelength ZooMed claims the tube outputs)
2)Iguana Light is the make, as Ford is a make of car.
3. 5.0 UVB is the model of the tube. 5.0 works for larger reptiles when the tube is 30 inches to 48 inches long.
4) F14T8/1L identifies the ballast in the tube's fixture. T8 is the diameter of the tube. 1L means one lamp. You can get fixtures for more than one tube (lamp), and their codes are different, but not hard for the lighting supply store to identify.
5. Zoomed is the seller who specializes in reptile products. Other manufacturers also sell UVB emitting lamps of various kinds.
Hope this helps.