Yeah, like Etl said, those are the stock covers that have been modified to fit over the installed roof rack risers. Those little pieces are going to be just as hard, if not harder to find than the roof rack components. A set of four sold for over $400 on eBay months back.
Take a look at my page here and you will see what we did:
http://www.nofuture.com/myvx/mods/roofrack.htm
And hell, since you are in Tahoe, you should swing over to Reno and hang for a bit with us Reno VXers.
Bart