If you can’t find local advertisements, you can look through social media. Some big ones are fridays for future and the a22 network. For some of them, you can also help remotely. You could also look for some kind of green party or ask some of the larger players whether they know anyone.

If you are involved in science, also take a look at scientist rebellion.

https://a22network.org/

https://fridaysforfuture.org/

https://scientistrebellion.org/