Ok, if it is an O-ring (and not a quad seal, etc) then buy the Metric and SAE set from Harbor Freight. Both should be $15. Every O ring in pretty much every airgun will be in one of the two.
If you want to be sure what you get is correct, go to the Crosman parts catalog for your gun ( probably
https://support.crosman.com/hc/en-us/articles/203544200-MARK-II-Owner-s-Manual-EVP-1966-1986- ) and call Crosman and see if they have the exact one you need. If Crosman sells it for $1, you can get it on eBay for $3 or an airgun vendor for $4.
Edit-just noticed the Entire gun parts list is not sold by Crosman any more for that model. Still, get the seal name, etc from the list for when you talk to vendors.