I have been using the bone saddle/nut shim material available from LMI for a few years now. This shim material is 0.02" thick, is real bone and is reasonably priced. These bone shims work great and easily cut with a fine toothed saw; do not use a knife, the shim will chip. Glue up with CA glue for a very strong bond. I suggest to always glue the shim to the saddle or nut and not into the saddle or nut slot, then sand and profile as needed. Sounds amazing, way better than cardboard, paper or wood.
Here is the link:
https://www.lmii.com/nuts-saddles/15...475-x-020.html
Be advised that when using CA glue with this natural bone material, the CA glue cures very fast, no accelerator needed.