Zack White sells practice leather for $15 for 3 pounds. Ranges from 6-9oz. I think I might get 2 orders of that.
Scraps are used for learning to do leather tooling and stamping (Basket stamping, for example). Scraps do not do much for learning to make an actual holster. would suggest buying one of the pre-cut kits for a holster from Tandy. Actually tooling, stitching, etc. would be more beneficial. After doing the kit, then you can experiment making a pattern from light cardboard (cereal box, etc.). and cutting your own leather. Use the cardboard pattern to draw with a pencil on the leather, cut out with a utility knife with a sharp blade. Pay attention as to which side of the leather to draw the pattern on...smooth or rough-out.