while changing my water pump on my 2004 Pontiac Grand Prix I lost one of the water pump pulley bolt does anyone know the size of those

That's going to hard to find that information out. I would just go to a junk yard and see if that have that year and get one from them...

And of course it the auto parts store can't get one the dealer can although if its a smaller dealership they may have to order it which could take a couple of days and you will pay 3 times as much, but then again its just a bolt. And in the short term remember it will run just fine with one missing as long as the other ones are tight. Just don't leave it that way forever.

Part Number 902-806 from Dorman. Just about any part store can get them. I ordered it from Amazon, and they will be here Tuesday for $10 for all the water pump bolts and the pulley bolts.