I usually get mine from Tandy Leather locally. Let's me pick it out. I've found that if you need to have it shipped, call one of the stores directly instead of ordering it online. If you talk to someone and tell them what you're doing, most of them are pretty good about finding you a good piece.
As for weight, I've used from 5-6 oz all the way up to 8-9 oz. Stay away from 8-9 oz unless it's a big chopper. Even them I prefer either around the 6-7 ounce. Plenty thick, yet easier to work with. No matter what weight you use, when you make the sheath put a welt (an extra thickness of leather to protect the stitching from the knife edge) in the construction. It is just another smaller, not thinner, layer where the stitching goes through.
Charlie