If you are an SMA member, they send out job postings via email as they are submitted. I have seen a lot of postings for the NY area, especially Showcases. The Equity website also has a job posting section- under "casting call", you can search for current SM notices. Opera America has a job posting section on their website but I don't often (if ever) see stage management positions listed.
I know of a couple of other websites for theatres and other arts positions, but they are in the DC area. I would imagine that there are similar websites for NY.
The other way that I hear about available positions is through an email list of stage managers in the DC area. I'm not sure who (if anyone) maintains it currently, but occasionally a job posting will make its way around the email list.