McMaster Carr sells a 14 mm O.D., 11 mm I.D., 1.5 mm thick silicone o-ring.
part number 5233T37
$9.75 for a pack of 50.
However, I'm not sure that the original o-rings/gaskets are sized appropriately. For the part from CM Becker that I am concerned with, the male threaded part has an O.D. of about 10.6 mm where the gasket slips on. The I.D. of the main ball lock fitting where the gasket rests is 15.1 mm. If I were using a round o-ring to make this seal, I would go for something like:
part number 5233T54
O.D. 14 mm, I.D. 10 mm, 2.0 mm thick, silicone
$10.13 for a pack of 50
I opt for silicone because it doesn't oxidize, which is what causes rubber o-rings to get leathery and not seal.
Now, addressing the failure of my gas-in, I've observed the following issues:
1.) The male threaded part that holds the valve in has "bow" in the flange that is supposed to hold the gasket. This may have come from stress relaxation in the plastic over time under the pressure of the gasket, or it could be a molded-in defect (warpage). Since this flange comprises 1/2 of the sealing surface, if it cannot achieve intimate contact with the gasket, all bets are off.
2.) The main gas in connector, in the interior female threaded part, has a "lip" where the gasket is supposed to seal. This caused the compression set damage to my gaskets. This is a defective part, either due to a defective mold design, improper mold assembly, or mold tooling not built to design. This interior flange should be flat in order to make a good seal with the gasket. It is not. The new connector I bought does not have this.
3.) The gasket does not center accurately on the male threaded part. This is because the I.D. of the gasket is larger than the O.D. of the male threaded part, near the flange where the gasket rests.
Attached are some pictures. I am still thinking about what the most economical, practical solution is to this issue. These connectors are used widely throughout the homebrew community, mostly without issues (I assume).
Bowed male threaded part.
Lip on flange inside main connector
Compression set o-ring due to lip on flange