Read the owners manual for getting the key fob to work with the car, its a very simple process.
Or search on here for the install instructions.
Buy from ebay or where ever, it just has to a 97-00 style remote, the 01+ remotes are different. So just search your year.
Just fyi to the OP, I had a bad experience with ebay fobs which is why I really recommend the website I linked you to. It's a toss up on ebay.
2003 Grand Prix GTP-3.5 setup, sold it, miss it
2002 Grand Prix GT-top swap, GT1 Cam, SD Headers, 90# springs, 42.5 injectors, 3.4 pulley - sold
2012 Chevrolet Cruze LTZ-Summit White, 1.4Turbo