Like trumpet, trombone is mostly taught within multi-instrument schools or conservatory prep. Below is the one studio that explicitly names trombone.
Real Boston-area teachers — dedicated private studios where available, plus the multi-instrument schools that have trombone on staff.
For trombone specifically, the conservatory and ensemble programs (NEC Prep, Boston Conservatory at Berklee, Longy, Rivers School Conservatory, Berklee for jazz) are stronger sources than Google Maps.
Trial class included with first enrollment — no risk, no payment until you're sure.