Try
http://www.billys.co.ukDepends on whether you have old or newschool frame, different seattube diameters. A lot of shims for sale are for new bikes and too big.
I found a Wald shim on Ebay that took 22.2 down to 21.1, it'd reduce your seattube diameter by ~1mm, but I noticed the seller is no longer registered.
It'll be available elsewhere though, it's a standard sized shim. It has a serial code if you need me to check it.
Edit>> I better mention the coke-can trick, you could make your own shim quite easily.