Most sheaths are made from 9-10oz veg tanned "tooling" leather. That isn't to say that you can't use other things and some do but that is what I have always used and made hundreds and hundreds of custom knife sheaths. If you are making simple sheaths and not needing them dyed or tooled you would be much better off with a product called bridle leather. It comes dyed with a heavy oil finish on it already. You cut out your pattern, edge, mark stitches and sew it up. It's nice stuff. Most people want to dye to their liking and tool them. DON'T SKIMP ON THE LEATHER!!! Buying cheap leather results in a crappy product. Knife sheaths are traditionally make from the shoulder or double shoulder cut of the hide. This is a relatively consistent section with bend and give to it. Don't concern yourself with stiff or flexible leather. It will all become floppy when cased (soaked in water) for tooling, molding or shaping. What you want is a tight grain side (the back or unfinished side of the leather) The fuzzier it is, the worse it is. Tooling leather is usually the highest grade in most leathers as it will not have the scars and marks of a regular hide. You need to find what's right for you and how you want to make your sheaths. You can certainly use a thinner grade of leather but it won't be a very rigid sheath unless it's bonded to another piece of leather. I sometimes do this to get a finished inside as well as outside on the sheath. For that I use a 5oz tooling leather. Bond both pieces rough side together, cut out and continue as normal. Just be careful that it is fully cured before bending, casing or shaping as it may wrinkle on the inside of the bend.
Grades of leather is important. Buy the best that you can afford to buy. It makes a difference. I personally only use "A" grade Herman Oak but I buy whole sides. I make lots of other things besides knife sheaths.
