The users handbook says that there are two 'O' rings on each wheel, one on the mounting surface on the inboard side of the wheel and the other on the back of the centre hub.
In your situation, much like the UK where dealer response is zero
, I would determine the section of the 'O' ring from the other wheel and purchase a length of similar material together with the correct adhesive.
There are other solutions to the problem, I did a search on www for 'o ring kit' and found the following UK link:
http://supaseal.co.uk/o-rings.htm
this is just one example there are probably plenty more.
Some of the products would not be cost effective for your repairs but your local suppliers may be able to direct you to users of their products. Hopefully you would be able to get the missing items manufactured [with a couple of spares?]
The seals will not have to be made to hydraulic seal specifications as their purpose is to keep water from entering the area where the magnesium contacts the steel.
If you do not want to go down this route go to gmpartsdirect.com and place an enquiry.
Let us know how you get on, good luck,