Go to Kentucky Leather and Hide and order one of their 3-4 oz deerskins. They are nice and soft and supple. If you only want to make a sporran, ask for a smaller hide. They run 8-12 sq ft per hide at $3 a foot. You can get 4 oz elkskin as well for $4 a foot. good stuff, although it sometimes has what look like a stain here and there.

Leather thickness is expressed in ounces, which translates to 1/64" per oz. It is typically sold by the hide but priced by the sq ft. tanneries use a laser to measure the irregular surface accurately, and you end up with a good bit of scraps left over.