I use 2-227 N-70. They are a little teeny bit to big but can be pushed into the groove and work well plus I have a local source where I can get them for something like 25 cents each.
O-rings through the dealer is robbery! The first and last o-ring I bought from the dealer cost me $9 plus tax and I think it was one for the oil filter cover.
If you have a Grainger close by take the cover to them and they can get one to fit exactly right. I hear they are very good.