Well you can either get a car cover through your chevy dealer or you can get one from california car cover (www.calcarcover.com)
1-800-423-5525. I just ordered mine from them, it was around $240.00 which included a lock and cable and a storage bag. Hope this helps.
I've always bought my covers from Covercraft which fit like a glove. I use the noah fabric (comes in gray only) here in Calif., it is soft and even protects from rain & sun really well. To get one go to http://www.bigskycarcover.com , I believe they are cheaper than going direct The cover from GM looks like it is the Sunbrella fabric (Black in color) which is very heavy duty and works great in harsher climates but to me it may be to rough on the paint. For info on different fabrics go to http://www.covercraft.com/custom_outdoor_covers.htm
BTW they have none listed under 2003 but are listed as 2004, here is the Part No: C16564NH . Big Sky doesn't have them on their site but call them for a price anyway.
The cover I ordered from california car cover was called dustop. This cover is soft for pampering the finish and thick enough to prevent nicks and dents if someone rubs up against it. I chose dustop because my truck will always be inside when covered so I did not need a big bulky cover for outdoors. As far as the GM cover in my opinion its not soft enough. Well hope this helps.