Extra thick soft temper, low stretch chrome tan leather is very rare and hard to get. I have been searching for a couple years to use making moccasins. If you are butt stitching like the Arrows then you will need at least 3mm thick. One option would be Horween Chromexcel. Maverick Leather Co. often stocks some of this in thicker weights. chromexcel is suitable for footwear uppers since it is a chrome retan and therefore pretty stable with wet dry cycles. The temper is usually medium/soft so reasonably easy to work with.
Another option would be chrome tanned Bison leather, this is usually very soft but it can also be stretchy if it is the "shrunken" variety. Distant Drums sells thick chrome bison in the US. Some latigo leathers can also have a medium/soft temper.