Wust Al Balad - If you don't mind to buy them second hand, there are a few shops in the Balad that sometimes sell them. One is close to Al Quds (the one on the way to Ras al Ain) @Downtown
@Virgin Megastore is cheap, but treat it with care or else it will probably break..Not the best quality. I've seen some @Downtown too, but just make sure you check the quality yourself before you buy!
Fasheh in @Downtown if it's still open. Near Jabal Amman and Shaheen service taxi. Lots of vintage frames, all between 10 and 30 Jd's that are made in Europe.