If you want to head up my direction, Pete’s Ace Hardware in Castro Valley has a great selection of washers and o-rings that are sold individually. FYI. They are limiting the number of people in the store allowed at a single time so I always see a line at the front. I’ve actually never been inside since the SIP order though.
Was going to say those look like the valves BRS sells, if you get their chat/customer service they might be able to tell you (or sell you) the o ring size