Some years ago, I was loading a .62 cal. smoothbore flintlock with 2Fg black powder and noticed as I seated the PBR that, what appeared to be, a lot of smoke was coming out of my vent hole. Of course I had cleaned between shots, so I knew it wasn't burnt powder smoke, but the shots over my chronograph were all over the place from a velocity standpoint. The next time I began loading, I closed the frizzen, loaded powder and ball, and found a full pan of priming-like powder when I opened the frizzen to vent pick the piece. This was a batch of GOEX powder I had purchased years before, so I ordered a powder sifter, and sifted enough "fines" to partially fill an empty powder can from several cans of the same stuff. The newer stuff from GOEX does not have the fines I found in the older stuff, and the quality is very good as well. That is my backup priming powder when I cannot get 4Fg powder. I also know that you can grind the grains of courser powder using a wooden spoon/wooden bowl or a stone mortar and pestel to produce fine grains for priming. Just keep sparks from metal objects away from the process, and grind powder in very small batches as a safety precaution.
As far as buying black powder in NY, all you have to do is call Coonies (800) 713-6321 or Powder, Inc. (877) 833-1799 or any of the other suppliers and ask if there is a problem. They know the limitations for all of the states. I take orders from other shooters in my club and when I have an order for 25 pounds, I place it. It arrives via UPS within a few days and I sign for it, then distribute to those that ordered it. Some of the suppliers can ship as little as 10 pounds, but the freight and hazmat fees will be the same as for 25 pounds, I believe. The price per pound will rise for an order of fewer than 25 pounds. But the price will still be less than the substitute powders sold in the sporting goods stores. Good luck!