Honestly as bad as it sounds I would start here
https://en.wikipedia.org/wiki/Slavic_paganism
and go for citations and sources.
Unfortunetly Slavic mythology was the biggest success of Christian Church goal to destroy anything non-christian so nowadays it's not popular, therefore not many people write/make movies about it. Most of our mythology doesn't even have any real sources

. And we don't have cool comic book heroes. I did love me some Czarnobóg in Nei Geiman's American Gods though.
There was one 2003 Polish movie named "Stara Baśń" (
https://www.imdb.com/title/tt0380726/?ref_=nv_sr_srsg_0) which takes place in the country of Polans in IX century if you are interested in actual movies.
As for documentaries I didn't happen upon anything anywhere on youtube or streaming services. And I would love to.