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I have purchased a number of items from jagmte.com and have been pleased with the quality. If you go to his site you will see his 'name' stands for "Just A Guy Making Things Easier".
The outer diameter of the part that it fits on is 55mm, and since you have a broken one, you should be able to measure the diameter of the ring itself. With those two numbers, you should be able to go to Grainger or McMaster-Carr and find an O-ring that would fit. That is what I would do.