I use 8/9 ounce vegetable tanned leather for my holsters, ammo pouches, etc.
One of the suppliers I use,
Leather Unlimited - Oak Leather
You'll need to learn how to stitch, mold, harden and dye leather.
I found a forum with lotsa info and helpful people,
Leatherworker.net
Before you start a custom leather project, I'd recommend that you make a mock-up of the finished product out of construction paper before touching any leather. Once you realize you can make your own custom leather products it can get addicting.