I buy my skirting leather from Wickett and Craig. I buy from them direct and they will level, split, or mill to my specs. I just buy whole sides, I've found if you buy backs it's almost the same price so buying whole sides you pretty much get the bellies for free.
For natural holster, strap, tooling leather I use Herman Oak, I just like the feel and the way it tools better than Wickett and Craig.