Bishop is frequently used as a resupply point on the JMT. We (locals) see a lot of JMT and thru-hikers hitching back and forth to the South Lake (Bishop Pass) trailhead. Many locals will pick up backpackers. It is also possible to get a ride out to Bishop with one of the many day-hikers who park there at the South Lake trailhead. Besides, the Bishop Pass trail is a lot easier than the Baxter Pass Trail (which is not in good condition).
There is a Von's supermarket in town, as well as Wilson's Eastside Sports, which has a good variety of freeze-dried foods.
Another option is this shuttle service:
http://www.eastsidesierrashuttle.com/If you can get others to share the cost, the Rainbow Pack Outfit is a very good, reliable service, and is a good source of trail conditions beta.
