Check out: https://www.macovidvaccines.com
There's probably close to 800-1000 appointments available there right now.
Also follow @vaccinetime on Twitter.
Right now about 80% of the over 16-18 year olds are eligible the remainder become eligible on Monday. There's been lots of appointments for the last week or so despite the J&J setbacks (MA had a big drop of J&J from the Feds recently and consequentially had to cancel & rebook some big vaccine events).
Also, because of the way the MA state reservation system works -- they give you 25 minutes to fill out the appointment details once you "claim" a slot, often times you'll find a decent number of timed-out appointments showing up after the fact. Not to mention there are plenty cancellations (as presumably people are booking for family or maybe just vaccine shopping).
It's all pretty dumb and arbitrary to me, the way this thing is being handled. In each case they were unable to get theirs locally, everything was booked with no insight into when the next availability would be.