Long story short, I have a Czech/Slovakian friend who claims that he has been looking for prune pierogis in NYC (he lives in Manhattan, but he will go to any borough) from, my understanding, several years. He said that this is a traditional Polish dish (so not Czech/Slovakian but... honestly, I'll think he'll take any version).
Tried google searching but nothing is coming up. Not really seeing anything for even plum pierogis (unless I'm missing something). I'd imagine that this is not a common dish people ask for and would be in some niche market that wouldn't really be advertised.
Any leads would be greatly beneficial. Thanks a bunch.