LRS and AM are the most common vendors for Mustang rims. Just watch out for the canned wheel/tire combos. I saved a ****ton of money building my own rim (from AM) and tire (from tirerack) combo myself not to mention they have some whack sizes selected for their pre-mounted combos so there is that too.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.