You can also get some O-ring material and make your own O-ring to fit. That's what I do. Use super glue to connect the ends. If you are thinking about doing this commercially and can't find a pre-made one to fit then you can custom order them to any size you need but they will have to be in quantity.
Also if your using a CAD program click on the O-ring groove and select properties and it should (I think)tell you how long it is. There is other ways in ACAD2000 but I can't remember them off the top of my head.
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