There is a small grocery and liquor store (actually pretty good wine selection) in the basement of the Mangy Moose. There is also the "Bodega" at the edge of the parking lot .. it had a different name and theme last time I paid attention, but has always been some sort of market.
Between them you can probably find, if not what you want, at least something that can substitute for it.
But a real grocery store to start the week is a better foundation.
In 2012 at least, the Hostel had shared refrigerators in the basement. I'm guessing they still do. The only problem is that they unlock the common areas for the morning fairly late. (I like to eat breakfast, then get ready, as opposed to vice versa. And I was in Steep and Deep, so had to be ready well before opening time. For a Gathering, the Hostal common area timing will probably be fine.)