There is someone selling a set of 04 Mach1 front seats on e-bay right now. Just do a search under Mach 1 Mustang and it should come up. Pretty good price too.
Been there done that, nobody in the aftermarket sells Mach seats. Ford only has seat parts available no complete seats. You either have to find a wrecked Mach, a Mach owner that switched to racing seats, or hunt eBay.