At the brewery. Here's the hours and what's currently being filled: http://jacksabbybrewing.com/blog/tourstastings/
I live here, but I've only been here six months .... I'm not a growler guy normally but I'm heading through Framingham tomorrow so I wanted to make sure.
Prices were from $9-15 last time I was there depending on the beer (most $9-10, but the specialty barrel beers were $15) with a $3 deposit on the growler.