The main shuffle stacks I use are specifically spares from four different custom sets from ASM/CPC. They're ones that are a bit off (wonky spots or offset inlays) where I'm not thrilled to have them as part of the set that sees play. A new-ish stack that I've been shuffling are "reject" Terrible's St. Joe's $25 where I bought two barrels when I only needed one, but knew I'd want/need to condition sort for the ones I wanted as playables in what I was building.