oddbean - others may say otherwise, but notice that this is not high temperature resistant like silicone.
https://en.m.wikipedia.org/wiki/Nitrile_rubber
Silicone ones will do the trick…(same ID and thickness as the one you found)
https://www.ebay.co.uk/itm/7mm-ID-x-1mm-C-S-Red-Silicone-O-Ring-Choose-Quantity-7x1-New-Metric-/293655938729
and in the mean time… PTFE tape is your friend. A few turns around it and don’t screw it too tight. I used to do that with the La Pavoni quite, as they don’t have o rings.