run to the hardware store (ACE, Home depot, Menards, Lowes, etc) head to the plumbing section. buy a few plumbing O-rings. (should be like 50 cents a pack) that look to be the right size.
Fit on the injector, until you find the right size,
once you find the right match.
head to the web. McMaster/Carr. High temp rings.
done