From the 800 bluebook window sticker web site
You must be an authorized Hummer dealer to purchase Hummer Certified window stickers. Hummer designated BAC number must be provided when placing order. Orders will only be sent to the addresses of registered Hummer dealers.
www.800bluebook.com/products_stickers_home.shtml
I've dealt with Window Sticker company at car shows. They make GM stickers for Corvettes and GTOs and their CEO is a friend who tells me "no one has rights for Hummer, Saturn, Buick, or Oldsmobile repro stickers" right now. Stickers are controlled pretty tightly by the federal government and FTC because of the possiblity of fraud (especially in older Corvettes and Mustangs - since there are more 1967 427 Vettes out there with "factory stickers" and build sheets than Corvette ever made - the same goes for Boss Mustangs)
http://www.window-sticker.com/
Fraud got so bad with old Tempests being made into GTOs that GM allowed PHS to manufacture stickers for GTOs after prodding by car clubs and the FTC. Pontiac Historical Services is the gold standard of window stickers. You can even order one on "original glued paper" for your old Pontiac.
http://www.phs-online.com/
Unless you are a dealer selling Hummers you're out of luck!