I see these out on eBay every now and then. It should be a 25.0 diameter size, not real common or interchangeable. Some Graftek restorers like to get these and take them to a machine shop so they can be turned down to 23.8 diameter (Graftek size). My best advice is to try to figure out the brand (by looking at all the seatpost pictures on Velobase.com) then search eBay for that brand with the "Completed Listings Only" checkbox checked under advanced search. Ignore the results that did not sell, average the sold price on the ones that did, assuming the component condition is the same.