Hello I asume you put the one you have on shifter for the rear derailluer, so you can atleast ride. that part should be fairly easy to locate. is it flat or convex? a suntour part may work.
try this site
http://www.ochsnerusa-store.com/search.php
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk