Dont know of any airbag refurbishers other than the sleazy car rebuilders that sell cars with "empty" airbags. They obviously get their covers from somewhere. I personally would not be comfortable cracking the seal on one to put another cover on it for the saftey end of things (exploding airbag) and for the liability aspect (if you ever sell your car). A used bag from the most recent compatible model would be the best bet.

If your brave you could test fire the old bag to see if it still works. I would suggest extreme caution doing such because they put out a lot of force. Long wires and a safe distance would be reccomended for such a risky but fun thing. Im sure the resounding explosion sound would put your mind at rest over the 15 year replacement term