Where are you putting this shim? I'm a little confused about it's purpose. If the pedals, even after tightening, were still loose in the crank, it sounds like the crank threads are stripped. You don't necessarily need a new crank to fix this as you could either buy a kit to repair the threads (Helcoil or other type of thread insert kit) or have a shop do it (though this might cost the same as a new crankset).
If you could explain the shim a little more I might have another suggestion.
If you did end up needing to replace the crank, I believe that you have a basic square taper crankset on the bike in which case any non-Campagnolo square taper crankset will fit. Just find one with the same gearing as your current one.