The proper shim would be 0.1mm (0.004"), half the difference in the diameter.

Aluminum pop cans are cheap, plentiful and the right gauge (at least the local Pepsi cans are), however I'll mentions some other options, for future reference. Shim stock is available that thin, though you may have to go to an industrial supply house and they may not sell you a small piece. Often, machine shops have a good supply of shim stock in various gauges and are willing to sell (or give) you a small piece. Finally, some automotive shops sell individual feeler gauges, which make excellent shims, but may not be large enough for some applications. In this case you could use two, placed side by side. Feeler gauges are excellent for shimming things like loose crown races and headset cups.