At Harbor Freight.
They sell O-ring kits.
They come in 2 types. American and Metric.
They're not real expensive. But worth every penny if you use just 1.
I bought a used diesel compression tester.
Tried to use it, wouldn't work???
Checked it out and saw it's missing an o-ring.
Pulled out my o-ring kit and found the right size and installed it.
Worked perfectly.