Do you have any furniture factories in your area? That' the first place I would go or at least call and ask them who their supplier is. I've purchased several pieces from a furniture store that deals only in leather furniture in my area.
9/10 oz veg tanned is the best I've used for sheaths. I can't remember the last place but I get it locally here in town. It may have been Tandy but it seems there was another place at the time. I got a large shoulder piece that has lasted me a long, long time. There is plenty more on it so I won't have to go look again for some time.
Oh, and on the stitching, I use a 5/64th” drill bit to make the holes and then stitch it up. If you get one of those leather tools with the wheel that ticks off the marks, you can just follow those marks with the drill bit. I do it in a drill press. Wet it some and when you are done it will all close up nicely.
You need a welt in there to keep the blade from cutting the stitches so you will be going through three layers of thick leather.
There are lots of quality leather suppliers. Just look for full grain leather that has not been split. The weight is in ounces. I like chrome tanned and especially latigo. Vegetable tanned is also good.
A furniture store that sells leather furniture might have leather samples available when they change out their inventory. Perhaps an upholstery shop would have some of that kind of thing around, as well.