[Crex]

Agree with Jim on buying leather. I have gotten some very good leather from Tandy but I sorted through several stacks of sides to find them. Unless you are sure of what you are looking at and have the time it's a bit frustrating to go this route.
Tandy does offer a few beginner courses in most of their shops if you have one close and may be worth going for learning some of the simple basics of cutting and stamping. However, you'd probably be better served garnering a visit to a sheath/holster maker in your area and get a little hands on experience.
Yeah, bellies are stretchy and can get baggy or distorted when tooled. Best to leave them alone while you are learning your skills.
I would recommend getting Tandy's catalog and basic Leather Crafting book for quick in-shop reference. But, I'm old school and like hard copy.