Unfortunately, I'm a city boy (life goal to make some money and buy some rural land). I've never had to think about where to get leather. My GPR came with some crappy naugahyde (those poor naugas!).
Where do y'all get your leather?
The easiest way to get it for me is ebay. I just ordered some pieces of 2-3 oz upholstery scrap that I'm going to make some small drawstring bags out of. Five 6"x9" pieces were $8.99 with free shipping (+tax of course). That's enough for a whole lot of flint pads. You can find different weights and sizes if you look around, too.
Btw, a friend used to have a t-shirt that said SAVE THE NAUGAS! BAN NAUGAHYDE! I always got a kick out of that.