If it's time for you to change to a smaller shim and you don't want to spend ~$45 on 4 new shims, call some of the local motorcycle shops. Most motorcycle shops are willing to trade shims for a small fee (cost me $1 each). They usually have a caliper to measure the shims, so you know you're getting the correct size.
This tip is especially valuable to those who have the East and West valve kits, which are flush with 270-275's, and devoid of 255-265's.
Cycle-Re-Cycle sells shims for $4.95 ea.
http://www.crc2onlinecatalog.com/Index_Main_Frame.htm (http://www.crc2onlinecatalog.com/Index_Main_Frame.htm)