Ditto to these gents'...
These have to be the best sources for leather and can find some good deals as well on ebay.
For a single time project ebay or a local Tandy may be a good start, but then once you do......you may be hooked and finding yourself searching the bulk rate stores like Mike and Dano posted!
It's intoxicating, and you're never the same after...
!
There are different tempers for different leathers so it may differ depending on which type of leather you like to use or go with.
You want somewhere between 5-8 oz...again depending on the specific tanned type. 5-7 oil tanned bison hide is my favorite to use in a medium temper. but if using vegi tanned or latigo cowhide, 4-6 oz. It is durable and yet plyable for comforming to your back.
Can't add much more to what has already been said. These guys know what they're talking about.