A list of proxy lists: https://proxy-scraper.de/sources/

The lists generated by this tool can be useful as well: https://github.com/maximilianfeix/proxy-scraper (which point to here: https://github.com/maximilianfeix/free-proxy-list)

There is also this bot proxy IP list: https://github.com/antoinevastel/avastel-bot-ips-lists