Try to coordinate on the ground at FRG. If you can't for some reason, try calling NY Approach on 125.70 (ROBER sector) for a Bravo clearance. The frequency really depends on where you're headed, what altitude you're at, how busy it is, and what configuration is in use, but usually the tower can recommend a frequency for you at your request. Altitude will normally depend on the runway configuration at JFK/LGA/EWR. I normally request Class Bravo at whatever altitude they can take me at since it's usually easier than climbing above, ducking below, or going around.
There are a few N90 controllers on here that may be able to share more operationally specific details.