Check out this web site. It lists all of the photo galleries in the City and gives you an idea of what they are showing.
Most of the galleries are clustered in the area between 10th and 11th Avenues, and between 18 and 26th streets. One effective approach in this area is to just walk around, stroll into the galleries to see what they are showing. Note that this is a former warehouse district, and it may be necessary to walk into a building to examine the directory to see what is actually there. Also, be aware that many are on a summer schedule - limited hours and perhaps second-string shows. The galleries aren't all photography - but it doesn't hurt to look at other forms of art.
There are also a few galleries in the few blocks North and South of Canal Street, and along Broadway. In the last few years, a few have relocated to a couple of buildings on 57th Street.
One of my favorites is the John Stevenson Gallery on 23rd Street, just east of 9th Avenue.