You could get the o rings sized from somewhere like Flow seal in Portsmouth for instance. If you can get them off most o ring distributors will be able to do this for you. If your after urethane o rings you'll need to know how hard they are (Durometer - 30-90. Higher is harder) Most Urethane o rings for paintball guns seem to be around 90 durometer although 70 might do it, but might be a little soft.