https://www.amazon.com/Bettomshin-Buna-Nitrile-Replacement-Instrument-Accessories/dp/B08VS63SNP/ref=sr_1_3?keywords=o%2Bring%2B50mm%2Bid%2B58mm%2Bod&qid=1643147876&sr=8-3

This is the one I used. However, there are many vendors. Just search on amazon "O-ring ID=50mm OD=58mm CS=4mm".

Plumbing supply shops probably sell them as well (and possibly some hardware stores).