I suggested to a lot of people that North Shore Thrift might be a good place to obtain a used/second-hand bicycle. I went by there this morning, and they are currently out of adult bikes. They will get more eventually, but don’t make the trip there for a few days, or you might be disappointed.
Craigslist is another place where you might find a good deal on a used bike. Try that.
Any other suggestions on good places to find used bikes? Leave them in the comments.