I don't know if the green ones in the pic above are original or not because mine were also clear (slightly yellowed from age).
I went to my Sears Hardware and tried
3/4 x 9/16 x 3/32
o-rings, but the weight of the pump caused it to contact the mounting base:
https://i.imgur.com/7OANV2n.jpgI then tried 7/16 x 11/16 x 1/8 and it looks perfect. Notice the gap between the base and the pump:
https://i.imgur.com/Xj26zWr.jpghttps://i.imgur.com/a1mhIgw.jpg Noise/vibration seems the same or less than with the original o-rings. I'm not surprised since the originals were pretty hard and did not return to a round shape when removed.
dcaudio, I'd buy a set and ship them to you, but they have less than four left! Let me know if you don't have any luck and I can check back later to see if they re-stock. They are pretty bad about restocking, though. You can also try McMaster Carr. They have a huge selection of different materials in this size.