Microcenter.

Microcenter has all sorts of computer products and accessories. The easiest way to buy things from them seems to be to front the money yourself, and then submit the receipts to get reimbursed. Make sure they don't charge tax on what you purchase; usually telling them it's a purchase for MIT and showing them your student ID is good enough, even thought they're technically only supposed to take staff IDs. Their telephone number is 234-6400, and they are located down Memorial Drive, about twice as far from Massachusetts Avenue as the Tang dormatory is.