If the correct o-ring size is not in stock, an o-ring can be made from round cord: cut to length and glue the ends with instant adhesive (Loctite 406 is named as suitable). Alternatively, o-rings from cord can be vulcanized at both ends in production. Round cord is supplied on a roll or cut to the required length. The cord-ring page describes making rings from cord, distinct from selling cord stock alone.