Please note that by some accounts,
too much UV light can be harmful to birds, just as it can to humans. A poster on another forum (the
parrot_lovers community on LiveJournal) has a UV fixture for her caiques' cage, but though she was initially happy, the birds developed skin irritation. She reported improvements after reducing exposure time from "all the time" to "a couple of hours at a time, a couple of times per day" (sorry I have no more specifics at hand!), and getting a larger cage that allowed the birds to move out of the exposed area when they so desired.
Edit/addenda: Note also that "UV" may refer to UVA, UVB, or the entire ultraviolet range of light; I
think that you would probably want something that spans the whole UV spectrum. (Whether something that's
marketed as "full-spectrum" or not
does cover this entire range is something you should look up. Whether I am
right is also something you should verify.

) Finally, be aware that UV light is filtered out by glass -- this is, after all, why your birds don't get all the light they need through ordinary windows! -- so make sure that whatever fixture you get doesn't cover your expensive UV tube with an effective filter.