This is probably one of my only real complaints about Rice. There aren't really any "convenient" convenient stores that are especially close to the residential areas of campus. That being said, there's a CVS in Rice Village (there's a bus that goes to the new graduate apartments if you don't feel like walking or riding your bike), there's a Walgreen's near Memorial Hermann Hospital (within walking distance... should be accessible via light rail, but I've never been there), there's a VERY small convenient store on campus in 13th Street in the RMC (so small that I doubt they have shampoo, but they might... lots of snacks and a few OTC drugs, though), and, lastly, there's a bus that takes students to SuperTarget (about a mile or so south on Main St.) on Saturdays.
I really wish they would put a Trader Joe's or something in the Med Center next to the south colleges or somewhere on Sunset near the north colleges (Leebron's house would be ideal, actually), but that's simply a wish... Trader Joe's doesn't even operate in Texas.