After a little searching, I found a couple of resources you might find handy:

https://indivisible.org/groups - find activist groups near you
https://givingcompass.org/nonprofits - browse nonprofit social justice groups you can join
https://www.globalgiving.org/ - browse global crises that need funding
https://www.astartingpoint.com/ - tangentially related, this organization focuses on discussion of contentious ongoing issues