I have a pair of Cobra-style headlights I got on eBay (dirt cheap, too) made by a company called DEPO. They fit pretty well, but you may wind up having to adjust them a little to get them to fit flush in there. On the plus side, doing so is fairly simple on these, since they use the exact same mounting hardware as the stock 'stang headlights.
Oddly enough, they fit my car better than my co-worker's genuine Cobra headlights fit his genuine Cobra. His stick out pretty far, and look kind of silly that way. Don't tell him I said so, though...