What kind of bike is it on? If its a Schwinn you can get one from Sheldon Brown I think? Bike Parts USA sells a bushing for a buck 98 that turns 13/16" into 7/8" there is a guy on ebay that sells the seat post that is 13/16 and 7/8" on the top like you need.
Last edited by stringbreaker; 08-16-07 at 10:11 PM.