Take a look at and you'll find reviews on several mini-maglite conversions (and numerous other flashlights as well). I have one with the Opalec and one with the Terralux--both work fine, the Terralux is brighter but with shorter battery life, the Opalec is not quite as bright, but runs a long time and holds full brightness until the batteries are pretty well gone. The Opalec also seems to give a somewhat smoother light field.
has both and I've done enough business with them to feel that they are a reliable outfit--the only time I've ever had a problem was right after Katrina--I didn't realize they were in Louisiana--anyway that order was slow but I can't really fault them for it.
You can find the Nite-Ize conversion at many sporting goods stores and the like--it's cheap and it works but it leaves a bit to be desired when compared with the others--the flashlightreviews discussion of it will give you the details.
Someone mentioned Surfire. Surefire makes good flashlights, there's no question of that, and if your life may depend on the thing working then that's t he one to go for, but you can get a whole collection of converted Maglites for the price of one Surefire and as for "purpose made bulbs" they use the same Luxeon LEDs as the Terralux Maglite conversion. There's also the battery issue--Surefires use CR-123 lithium cells that are hard to find and quite expensive compared to the standard AAA, AA, C, or D cells that the Maglites use.