When I design a pair of mocs, the style and pattern strongly suggest the type of leather I would use. For most soft soled styles, elk or moose. Buffalo (American bison) is nice also...the toughness of bull hide but doesn't dry out as hard, but firmer than moose or elk. If hard soled, a good grade skirting or tooling leather is nice-veg tan. I takes a shape nicely for a contoured sole. If you are a purist and have deep pockets, use a brain tanned leather-$$$$. It is a variable product and quality very much depends on who's elbow grease has been applied. In my experience, it can stretch too much, dry too hard and it's around 3x more expensive. It is nice to work with though! Chrome tanned leather is a modern wonder. The 'German tanned' leathers are, as far as I know, a type of oil tan and have a very nice bead-ability similar to brain tan. I have not seen very heavy hides on the market. It seems to be used for garments mostly. Correct me if I =m wrong, I would love to find this tannage in a 7-8 oz weight!
I know a place selling a nice moccasin weight elk or bison for $6.50 per sf. half hides and quarters.