www.smallparts.com any O-ring they don't have probably doesn't exist! Remember to pick a material compatible with oil, such as Viton or Buna-N. Silicone is not a good choice.
I used to race 1/8 Scale unlimited hydroplanes a couple years ago. The tuned pipe had a Hi Temp O-Ring that was resistent to oils. I'd bet if I could get a handful of those in the right size I'd be set for life.
Joe