I measured one of the o-rings which AoA sent me a little while ago for my Bantam Sniper Magnum.
As I expected for a .22 cal, the I.D. is 5.5mm so my only question was the thickness or cross-section, which I measured at 1.5mm. So a pretty standard size - 8.5mm O.D. x 5.5mm I.D. x 1.5mm cross section. You can get them from an o-ring specific store, or even Amazon.
@tmh, has provided the actual dimensions. I checked with my dial calipers, good luck. I have four of them, if you need one in a hurry, PM me your address and Il'll drop it in my mailbox in the morning.