Basically you need the HID projectors and ballasts and bulbs from an OEM vehicle, and then you retrofit them into your choice of Mustang headlight housing.
I beleive TL projectors are one of the more popular to use because they feature a flapper which gives you a functional high beam.
yeah.... that's a retrofit... i did mine on my last coupe... turned out really good... only wish i had gone with ultraclear headlights... the smoked lights really hurt the output, but believe me, they were still definitely bright enough... i used a bi-xenon set of projectors from an e46 bmw. ask mustang5l5 tho... he helped me on my set quite a bit.
I want to do it so bad, if I actually start driving my car more I think I'll do it. Do you do better finding parts on Ebay or something like that HIDPlanet site? I've got to retain high beam living in VA so I'll have to get a set of bi-zenons fo sho.
Check ebay, but once in a while the guys in HID planet clear stuff out and offer cheaper. If you aren't in a rush, just wait til something good pops up