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First, contact Gene or Patrick at GMPARTSHOUSE. You get real knowledgeable people who answer the phone. They should be able to provide you with oem GM bolts if they're still available (or available separately)... http://www.gmpartshouse.com/
If that doesn't pan out, try McMASTER-CARR. You'll need to know the exact specs. They should have something very close to oem bolts... http://www.mcmaster.com/#
Just went through this so I wanted to revive an old thread with the updated info. I Couldn't find the bolts online and the parts numbers above are either discontinued or wrong. I went to a chevy dealer and order them. The parts numbers are:
Bolt: 10095393
Nut: 11609746