Hornet o ring

[ikbtops]

Hi all,
I finally broke my last rubber o ring on my Hornet.
Is there anywhere I can find these? Either online or otherwise.
What size would I need if I looked at a hardware store?

 

[ikbtops]

Okay, so I managed to cut a thin enough cross section of a cartridge condom to slide into the groove. I'm not sure how well it will do preventing leaking etc.
If I can buy something closer to the original I'd like to though.