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You will have to research this yourself, but I'll get you started.
You may find remotes on ebay that are quite cheap. I got one for $15 and it works fine with my 2001 C5. But I seem to remember that they changed the remotes some time prior to 2001 and the older ones are more expensive. Not sure if the new ones work with the older car, but pretty sure the old ones dont work with the newer car. So try and figure out which ones you are looking for before you buy them.
When you locate the remote that is right for your car, programming it is easy and will take you like two minutes (thats only to have it adjust the seat/radio/and steering wheel by the memory position you program the remaote to). Its done from the options menu on the DIC.
The dealer has them, when last I asked they told me $128 each. You should not have to pay close to that no matter which one you need.
How long ago did you buy the ones from the GM Dealer? If you got them less then a year ago they are under "Replacement Parts Warranty", or over the counter parts come with a 12 month warranty. If over a year make sure the batteries in them are good, and if so have the system checked to make sure it is in good order.