Here's the link to the company that manages many of the farmers markets in Westchester. They list locations, when they run, and vendors.
Some towns, like Bronxville as of last year, are now taking over running their markets themselves. I don't know whether this will be a good or bad thing, I guess it depends upon the actual person managing it and whether they are seeking out great products.
I agree that the one in Ossining is one of the largest and best, also one of the oldest.
The one in White Plains on Wednesdays in the parking lot off Main St. is mediocre.