Depending upon which type of bottom bracket you have? If you are looking for a freewheeling one piece crankset that fits the large BMX/Cruiser bottom bracket you need to search the various vintage Schwinn/Cruiser sites. Schwinn made a freewheeling one piece crank set and you should be able to find one used at those sites. If you are looking for a freewheeling crankset to fit a contempory european bottom bracket then they are currently available thru a company called Cyclone-USA http://www.cyclone-usa.com/store.php?crn=203 scroll down the page to find the various pieces to put together a freewheeling crankset.
