In Patten:
Ellis Market is a full-on small town grocery store. You can get whatever you need there. I'd stop at filet mignon and morels but otherwise you'll be fine.
There is a diner/pizza joint, too, although I'm usually disappointed with the meal. The hardware store across from the grocery has white gas and other basic camping stuff you might have forgotten.
Closer to BSP:
For local culture and last minute items, you can stop in and Joe and Sue at Matagamon Wilderness Camps just before the BSP Matagamon gate (right at the crossing of the E. Branch). They have the bare essentials and will serve you fine sandwiches, hot coffee, cold beer, or whatever else you fancy. Plus they tell good stories and are awfully nice people.
When we talk about non-motorized recreation contributing to the economy, the motorized folks shout from the roof tops that we (the non-motorized) don't actually spend money. Anecdotally, they are largely right so don't be shy about dropping a bit of coin in a place that supports the landscape you love (or will soon love, if you haven't been there).
If you want some data and info regarding tourism spending and non-motorized recreation in the area, look
here
... and have a blast!