In the female foster fitting on the hose? I asked the same question some time ago and the answers were less than definitive. Apparently they are not all the same (manufacturer dependent), but close....I had to experiment a little to find one that sealed but wasn't too thick to prevent a solid lock in with the male attachments.
I know that's not really all that helpful (even if I'm thinking of the right part). But, if you have a local source grab a couple and try. I know my local HD doesn't carry much in the way of small orings but my local Ace hardware has some, or grab a metric and US variety box (last I bought these were $9 each at Harbor Freight).
If it is the oring in the female foster on the hose, It's rare for these to fail unless the hose is pulled off with some pressure in the line so that might be the paintball shop...
