Solarpunk

You probably just want to get on the email lists of local organisations (activist groups, minor political parties, union councils, etc.).

They will often cross-promote each other so you'll be made aware of other organisations through them.

Gotta get involved with your local protesters. Look for DSA or IWW chapters to start with. College towns usually have a few groups.

i don't know if it exists but it's really easy to know about them before they happen: follow the politically active collectives, associations or unions of your city and you will find out. rss, activitypub (fediverse) and telegram are suited for the job because timeline won't make important posts disappear in favor of ads or posts with more engagement.

Sure officer… that way…

Now seriously, it’s possible a lot of protests are not organized openly to minimize risks. Just get involved in groups that interest you.

That's a bad idea because it could help police in surveillance and identification of protestors. As someone said, get yourself AND people around you involved in local community groups.