I always seem to be able to find fuel/oil resistant orings in whatever size I need from McMaster. I have their app on my tablet and start using the selector and my caliper. Iíll usually get a few above and below my measurements ďjust in caseĒ. I then put the ones I didnít use in my oring drawer with a tag as to dimensions.
Iíd bet that there is a hydraulic repair shop in Green Bay Area that may be able to help as well.