I also was asked the question on the PM--Beckson makes standard replacements for their 4", 6" and 8" O rings. As Boris suggests--there are many O ring supply houses. Many hydraulic shops carry a large assortment of "O" rings. We have one in Pensacola (Pensacola Rubber) which will cut any size square ring (work as well for many purposes, and often cheaper) you need. I had some older style sea strainers on one of my large boats. They made me rings in a couple of minutes.
If you have the exact size--then you can get a duplicate--I have digital calipers which I used to measure items like this. You can do the circumstance if you cannot get the diameter, with with a piece of string.--or in this case, I would use 1/8" line or paracord.
Use silicone grease on the "O" rings regularly to preserve them.