Do you mean the one that interacts with the co2 filler? If it is that super small o ring you can simply break a cheap 1 dollar lighter and get 2 of those o rings from there. Then use a propane adapter to push the oring onto the valve.
Ive tried the moon dog ones as well. The thickness and diameter is to much to fit in the fill valve.
If you have braces, or go to the dentist a lot you can ask for the small elastics they use for braces. Those actually work quite well too.
I made a thread about this a while ago.
http://airsoftcanada.com/showthread.php?t=157657