I don't know were your are from, but Princess Auto. has the O rings. Just for the couplers, or in sets up to 407 O ring for $14.99. They have all kinds of hardware kits with tractor stuff. This is the DIY store. Their on line, and they ship.
Dave
G'day as mentioned earlier there are alot of 'o' ring kits avail and most have the right size that you want if i am right the 'o' ring you need is 22 x 3.0 mm, if your dealer can't help you with something as simple as that, then it is time to find a new dealer. Best of luck