I found two sources (after a two-minute scan of the ads in Muscle Mustangs and Fast Fords). There are probably more but you have to do
some of the work, too.
Try :
www.50resto.com
They advertise a complete leather upholstery kit (79-93 Mustangs) for $699.95, a front-seats-only kit for $499.95
Texas Mustang advertises 'leather upholstery' starting at $799.95
www.texasmustang.com
This should get you started.