Its pretty much impossible to tell what you need from your description but here are a couple of things you need to know to spec an o-ring:

Inside Diameter
Outside diameter
Width
Shape (round, square, rectangular, x-shaped)
Material

Do you have the old ring? Would it be possible to order a replacement from the manufacturer? Or at least contact them for the specification; O-rings have specific spec numbers.
If all else fails, you can buy o-ring assortments from industrial suppliers fairly inexpensively. That might be the way to go.

 

 

If there's any movement in the o-ring fitting, like in a quick connect fitting, it helps if you lube the o-ring. You can use Vaseline but a better choice would be silicone grease otherwise the ring has a tendency to get worn. If its a stationary fitting the lube will help there too. They leak because after a while they tend to take a set and no longer exert pressure on the assembled parts which is what makes them work as a seal.

 

Measure each you try so you know your size for next time. Viton is preferable for chem resistance (it's also used for the tips of carb needles) but buying a kit of Viton rings could be expensive. BTW if the washer was not made in the US any SAE seal or ring matchups are nice but accidental. Try to measure in metric for metric machinery. The cheap Harbor Freight digital calipers work surprisingly well (and save my Mitutoyos and Tesa for shop use) and have pushbutton swap not just between thousandths and metric but also display fractions.