Borrow a pair of vernier calipers. Take an o-ring out (assuming both the same). Measure internal diameter of groove. Measure Width of groove.
The O-rings you'll need are sized ID (internal diameter first) so for example if the internal dia is 24.00mm you'll need a 24ID O-ring. If the groove is 2mm wide, you'll need approx a 1.8mm section o-ring, maybe 1.7mm.
So you'll be looking for 24mm x 1.8mmm O-ring. BTW, BIGGER section is NOT better. An o-ring only needs to be compressed 10% (so a 1.8mm section gets squashed to 1.68mm) to do its job perfectly
If you have a 2mm section o-ring that's 20mm ID that will also possibly fit as they stretch a very long way. However, it will not last too long and will eventually snap. It may however get you out of a leaky hole in the short term.
ALWAYS leave a gap in the o-ring groove when the o-ring is fitted. Because as the o-ring is squashed in, there must be somewhere for the squashed material to go away. Having nowhere is why you cant get what you have to fit. Push hard simply slices chunks off the o-ring.
When fitting, use SILICONE grease to lube. Never petroleum grease or similar.
You are also looking for either Nitrile or EPDM material. EPDM is more robust, Nitrile cheaper. DO NOT use VITON. It's unsuitable for potable water.
