Leather is produced by several different methods of tanning. Brain tanning uses the animal's brain in an emulsion. Urine and feces can be used for tanning hides. Chrome tanning utilizes chemical salts to tan hides. Vegetable tanning uses tannins found in plants, typically tree bark but there are other sources (including tea).
Generally, the only type of leather suited for holster making is vegetable tanned which is capable of being formed and retaining a shape. Vegetable tanned leather is generally stiff, while the other methods result in leathers that are soft and pliable.
Vegetable tanned leather is commonly referred to as carving leather, oak tanned, or similar marketing terms. It is available in different thicknesses, commonly referred to by weight in ounces. One "ounce" is equal to approx. 1/64", so 8-oz leather is about 1/8" thick, 4-oz. is about 1/16" thick, 12-oz. is about 3/16" thick.
For most holster work leathers between about 6-oz. and 10-oz. is useful, with the lighter leather frequently used in holsters for smaller and lighter handguns and the heavier weights for larger and heavier handguns.
Veg-tanned cowhide from the shoulders and backs are usually very good for holsters, belts, and accessories. Sides are also very useful. Bellies should be avoided because of great variations in thickness, wrinkling, and loose fleshy undersides.
Much of the leather found in craft stores will be imported, and the tanning methods will not be known. I recommend the products of Hermann Oak Tannery and Wickett & Craig Tannery, both being US companies of excellent reputation. One good source is Springfield Leather Company (quick google search should put you on their website), where you can purchase Hermann Oak veg-tanned in smaller quantities such as a few square feet rather than purchasing an entire side or shoulder.
I hope this helps.
