Historically, I’ve just used veneer that I have lying around as shims. However, lately, I’ve been leaning more towards plastic shims I’m getting from http://www.handi-shim.com/
They come in handy all over the shop and there’s little holes in them that fit nicely over the barrel bolts that hold the cabinets together and you’re done. The smallest one they make is 1/32″ which is typically just about right. If I can’t use those, I drop back to veneer.
I hope this helps…