Two places come to mind. The first is the Weird Stuff Warehouse in Sunnyvale. You might not find what you're looking for, but you'll certainly come away clutching an armful of stuff that you never knew that you needed!

If you actually need good cable, and don't want to mess around, the ultimate cable place in the city is DIA Cables, in San Francisco. Nestled on the 4th floor of an otherwise unassuming office building on Market Street, this is THE place to find unusual cables and adapters. I've been going here since at least 1997, and have nothing but praise for the quality of the cables, and the knowledgeable staff. In addition to the stuff on hand, they make custom cabling. None of their stuff is particularly cheap, but you get what you pay for, as their stuff is consistently excellent. Besides... where else are you going to find a Sun monitor switch? :)