I found the oil filter cover O-Ring dimensions by buying a new GU90706444 O-Ring from the dealer and measuring it. The OEM O-Ring is an industry standard size JIS P45 (3.5mm CS x 44.7mm ID). I ordered some of these P45 O-Rings from an online O-Ring supplier and they are a match. I had to make a minimum purchase of $10.00 with the online supplier. Since these oil filter cover O-Rings only cost .25 cents each online I now have 40 of them. With one or two oil changes a year I guess I'm set for a while.
PS: I did something similar by buying drain plug and oil filter cover crush washers from Amazon, also pennies apiece.