I've purchased several OEM items from Ebay sellers for my Saturn SKY Redline and Saturn AURA XR.
Just make sure to check the seller's feedback rating and also see if he specializes in selling car parts or, in your case, Chevy parts. Double-check to be certain you're going to get the tonneau with the bungee cord AND the bolts that need to be attached. I think you're going to do alright by getting an OEM tonneau off of Ebay - most I've seen come with the storage bag, as well.
Good luck and please let us know how it turns out.
2011 Camaro 2LT RS Convertible
Build Date: 06/27/2011
Title Received: 08/15/2011
1984 Camaro (3rd Gen.): Traded: 1987
"Meet the new boss. Same as the old boss."