No offense, but, how much air do you need? I've never seen anyone use doubles in all my diving experience (23 years of teaching SCUBA in tropical climates). I don't know of anyone here who will rent doubles. Like it was mentioned, B&B has them but they are primarily used for the rebreather setups and deco stops. Maybe you should look at getting a steel 100 or something if you need lots of air. Sounds like you should try to get this figured out before you get here or you might find yourself out of luck in the double department. If it doesn't work out, you could just use gear that is available here. All the dive boats can provide gear for the dives you will make with them. Some charge extra for this, some don't. Then you wouldn't have to drag your set-up all the way from the east coast just for a couple dives, might make things easier on you.
Good luck and have fun. If you need info about the south shore dive sites, I hit them all the time and would be happy to give you a report on current conditions and stuff.
Aloha!