Harris cyclery sells them for about $25.00 I think. They are made by Tru-vativ. I used one on my continental and PastorBob has too. Like Sheldon Brown said " Be the first kid on your block to install Dura Ace on your Varsity" or at least thats the way I think the quote went. One thing to think about before you do the conversion is that unless is the main concern the old Ashtabula cranks were nearly bulletproof and you can still replace the sprockets
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(Life is too short to play crappy guitars) 2006 Raleigh Cadent 3.0, 1977 Schwinn Volare, 2010 Windsor tourist. ( I didn't fall , I attacked the floor)