I got mine from the dealer for $7.
I didn't notice anything special about it. It seemed to be a regular o-ring. Even if my memory fails me and it's a d-ring or a short tadpole ring gasket you should be able to get away with a simple o-ring from McMaster of the same diameter.
You should be fine with just Buna-N at A70 or A50. Check just find the approximate size.
http://www.mcmaster.com/#o-rings/=e5d3lr