Is there a place that sells cheese piroshki in nyc?
I can't point to a specific store, but perhaps someone will have knowledge of a place in Greenpoint with cheese piroshki? In addition, I think Union Market (Cobble Hill/Carrol Gardens location), Fairway in Red Hook and the Key Food on Henry Street in Cobble Hill have them, but would suggest calling to confirm before heading out. Good luck!
480 Van Brunt St, Brooklyn, NY 11231
288 Court St, Brooklyn, NY 11231
Unsure how different piroshki are from pierogi for your purposes, as well. Most supermarkets in Greenpoint--even C-Town (on Manhattan) and Key Foods (on McGuinness)--carry pierogis, including the cheese variety, though they are fairly professionally made. I don't know the best place to get the more amateurish kinds I like.
Outside of Greenpoint, it's hard--some stores outside of Greepoint only carry the potato-and-cheese varieties, which I kind of hate.