If you keep checking Playbill that's your best bet. However, I should warn you that a lot of theatres are union halls where you must be a member of their union to do ASM or anything else.
My recommendation if you don't get an internship from that list is to start calling NYC theatres and ask who promotes certain shows that you want to help out with and approach the production company about clerical or administrative work.
As far as modelling and talent agencies go you need to contact the Screen Actors Guild in the U.S. or ACTRA in Canada for a list of reputable agents as there's a lot of bad people and agents out there and you don't want to work for them.
Start calling the agents and casting directors they give you a list for in NYC to see if they need an intern. If you are part of a college program or just recently graduated contact your profs. as they might be able to help with leads. [ solidadvice4teens's advice column | Ask solidadvice4teens A Question ]
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