In a pinch, the black fuel line that some of the Plastic read to fly planes can be cheese sliced off to make an o-ring. I've done it at the field several times, even taking the fuel line off the plane we were working on, cutt off an o-ring, and still had enough fuel line to reach from the tank to the engine!
You just have to wanna!
Bernie has a real nice site and he was just featured in the current issue of Aero-Modeler Magazine.
Good luck and have fun,
Dan McEntee