Tandy Leather is probably the most known, if you have one close you can go there and pick your leather.
Google vegetable tanned leather for sale and you'll find different places you can order from.
I use 8 to 9 oz. leather for most my holsters and only vegetable tanned. Any chrome tanned can remove gun finishes.

As I mentioned before, the inside of a closed vehicle here in Bama in the summer time can exceed 125 degrees and unfortunately guns are left in them all the time. I say unfortunately because they get stolen from those vehicles. A co-workers was stolen last week.