A good number of good pros do Pro/Am, but the better the pro is the more expensive it will be, and the best ones have full lists of students anyway so you can't get lessons even if you wanted to. If it's a choice between not doing any dancing at all and dancing Pro/Am, then doing some Pro/Am isn't always a bad idea if you can hook up with a good TEACHER and not someone who will merely show you steps and tell you you're doing great but not really teach you anything. Of course, it's difficult for some people to tell the difference, but that's a whole 'nother story right there.
College clubs generally have good teachers and excellent prices -- even if you're not a student.
From time to time you can find adult dancesport group classes, there are several in my area (San Francisco) geared toward people who learned to dance as adults and want to move into competing. Hopefully you can find something like that in NYC. I'd say check out the more dancesport-oriented studios to see what they have on offer, like Ballroom on Fifth. I have no idea what they have, I just mention them because they're a name that comes up all the time.