There are a lot of books out there about illegal casinos. The ChipGuide has a list of some;
http://chipguide.themogh.org/cg_books.php

I assume you have already looked at Ed Hertel's price guide;https://www.chipster.net/illegal-chip-guide

If you look around that website, he has links to his many articles on specific illegal operations.

I hope that helps in your research.