You can also try contacting the NYU or Columbia film school departments...students shoot many short films throughout the year and they often need ACs. Most of the work will be unpaid, although sometimes they'll have a small amount in their budget. However, you might make contacts with some working DPs by working on those crews. Usually inexperienced crews and producers so it can be frustrating but if you're brand new to the city...
Local 600 is the IATSE camera local here in NYC. You can find contact info at www.cameraguild.com
If you decide to join, you'll need some sort of proof that you can legally work in the U.S. (citizenship, working visa, etc). It's not exactly cheap to join (around $4,500 for a camera assistant - 1st, 2nd or loader - payable in installments), you must make your own contacts in the business, they do not find you work and you will be expected to pay dues four times a year. However, they provide many training sessions and networking events every month, the rates and benefits are better when you do work union jobs and you can still work non-union jobs in between your union jobs.
Good luck and best wishes!