i'm a bit late to this thread, but for anyone still looking, here are some!
the Anarchist Black Cross is a good resource for political prisoners.
here's a guide from them, most of which applies even for "non-political" prisoners (if such a thing even exists). they have a bunch of chapters and i think theyd be worth checking out.
Black and Pink is also an organization that does a whole lot of support and penpal arrangements for specifically LGBTQ+/queer prisoners. They have a pretty extensive penpal guide
here, and in step 2 they have a whole friggin' database of people looking for penpals. it's wild.
for specifically political prisoners, there's a database called
PrisonerSolidarity.net that i haven't really checked out but looks promising, and itsgoingdown.org typically has posts for prisoner birthdays i think. (
here's the one for December 2018.)
i'm sure i'm missing a whole bunch of good resources, but that's what i got for now. hope it helps someone!