The metal in a soft drink can or a beer can makes a handy source for shim material. Just trim out a few with a small pair of scissors. Most professional-grade parts stores like NAPA will probably be able to supply you with roll brass for shims of various thicknesses if you need it. Pre-formed shims are nice, but not really necessary.
Edit: The Federal-Mogul Master Engine Bearing catalog used to list a part number for main bearing shims- - - -don't know if they're still available or not.
Jerry
Last edited by Hotrod Lincoln; 10/23/2010 3:29 AM.