You can take the O-rings to an industrial hydraulic hose supplier in your area. Look up American Hose and Fittings, see if they are around you. They are big, I have used them a buncha times for stuff (mostly custom hydraulic hoses and fittings) for my truck and cars. Or find a Parker hose distributor, they will sell O-rings as well (American is a Parker distributor). You might also find the O-Ring at a Grainger dealer, although your best bet is an industrial hydraulic hose dealer.