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.

FOSS will make it harder.

Emails of local groups that send out newsletters.

Local community groups and activists, easier to find on social media, though.

Some people I watch on yt include info. on protests in vids and what local communtiy help they do, not all yt people have orgs. though.

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.