There are a few ways to get PA gigs in NYC. Part of it is who you know, of course so making contacts with other stage managers is always a good thing. Most of the companies in the cities you can write to directly and work as a PA, many of them do pay a stipend. Some of the larger companies hire PAs through the auspices of their internship programs. Good companies for big shows to write are Roundabout, Manhattan Theatre Club, Lincoln Center, and The Public. They all use PAs on a regular basis. Go to their websites for more info, or call their admin number to ask who to address the letter to. I also tend to buy the Theatrical Index every few months to see what new is going on and send letters to GMs or the listed production stage manager. I haven't had much luck with writing general managers, but I have had luck with writing PSMs and theatre companies. Best of luck!