Back in October 2013, the infamous street artist Banksy did a one-month residency in New York, putting up new guerrilla art each day. Almost all of that art is now gone (removed, painted over, etc), but if you are looking for Banksy in NYC, one piece remains. On West 79th St, just east of Broadway, the building owner put a piece of plexiglass over one of those pieces, preserving it permanently.
I can help you find all kinds of street art throughout the city with a custom tour.
For information on Banksy-- images, articles, exhibition listings, and more-- see the detailed profile on this global artist on Artsy.net ....they are a fantastic resourse.