I would like the rails also. I don't understand why so many of these are missing the rails??

 

You would only be able to find the actual factory optional roof rack used as they are no longer available.
On many when the roof rails are missing it is because it would not fit in a prior owners garage with them on.
I assume this is the roof rack you are talking about. Its the only one I'm aware of with the Hummer logo.
Either way you will probably only find original rails used as well which are pretty common on eBay, Craigslist, etc. There are some aftermarket rails on eBay too which are pretty good but with a catch - the end caps do not lock, nor do they have any way to add the locks which is goofy since there is no way for the caps to stay on without the locks.