I bought a pair of those last year for under $10 on Ebay. They are actually pretty good working (though not elegant looking) shifters. It's easiest to just go buy another pair. Or, if you aren't in a rush, search around and find a nicer pair of thumb shifters. I picked up a bag of Deore XT drivetrain parts for $15 at a swap meet yesterday, which included some very nice Deore thumb shifters.

Velo-Orange offers them for $12. Not much choice in thumb shifters these days, but they wouldn't offer something that didn't at least function well. http://store.velo-orange.com/index.p...-shifters.html