Most of the local dealers have them, if you are prepared to settle for aftermarket. The genuine Beretta mags are better built but the aftermarket ones usually work OK. Try Four Seasons in Woburn.
The Powderhorn in Hyannis had a box of them the last time I was in a few weeks ago.IIRC they were ~ $15 each;not being very helpful but I didn't see if they were MecGar,Malaysian military or what-not as I was too busy digging through the best of the $5 1911 mags.
Rileys in Hooksett NH has Pre ban unmarked Beretta 92FS mags.These mags came from a POlice Dept that traded in its berettas. THe mags are used but in good shape for $20 a peice. Just sort through them carefully because some of the Marked mags are mixed in. I bought 10 and so did by buddy.