As with any university town there are expensive convenience stores located near the school. Wawa, a 24 hour convenience store located in the College Park strip mall just south of campus, is one of the best. Wawa has the most inexpensive gallons of milk around and also good coffee and deli sandwiches. You can expect to run into swarms of undergrads there on weekend nights.
Other alternatives are the Co-Ops (Co-Operatives). There are three near the University: the Greenbelt Co-Op in Old Greenbelt, a Co-Op in the Berwyn Heights section of College Park, and the expensive but fancy-schmancy Takoma Park Co-Op.
There are also farmer's markets every Saturday morning in College Park, Takoma Park, and Laurel. (Burtonsville, which is just west of Laurel, also features an authentic Amish Farmer's Market). These typically operate from May through October. They have great deals on really beautiful, fresh produce.