Win Aquarium almost always has then. Not sure of the address, its in Chinatown tho. Do a search on here and you'll find it as its been discussed a few times. They had $1.50 amano shrimp. They call them algae-eater shrimp tho, and they're mixed with other shrimp, so make sure you know what an amano looks like beforehand (tho the owner is very knowledgeable as well).
I did just find the largest amanos I've ever seen at a fish store in Astoria on Astoria Boulevard for $2.99. They're double the size of the ones at Win. But that's not Manhattan or BK.