anywhere that sells auto parts. basicallu you need a 7" headlamp but i bet youre looking for a part number? i got some badass xenon headlights from www.mustangdepot.com and i hear someone on eblah sells some for cheap too. here's a pic of mine before and after, guess which one is which.
IN the states they sell "Xenon Bulbs" its not really a xenon light system since those are sealed high discharge lights with a special gas in the lense (or something like that) when we are talking xenon lights in the classic stang its a basic upgrade (which I did) that replaced the old 7 inch sealed beam for a glass or lexon housing with either a straight up halogen bulb or a halegon 'xenon' bulb. They are certainly brighter than the old headlights, but not a truly xenon light.
Fifty bucks, no relays required. They look good and light up the road very well. I recomend converting to a higher-amp alternator and an electronic voltage regulator to assist the improved lights by supplying a steady, reliable source of power.