Two locations, one's in Dumbo and the other is on Ave C between 5th and 6th: http://www.recycleabicycle.org/

There's also an old shop on 6th between A and B (or B and C...pretty sure A and B) that has more old parts than new by the looks of it. Franks also on Grand street on the LES (few blocks away from the water). They should all have well stocked used parts bins.