This is the link to the 1970-1977 165 outdrive gimbal housing, but the pic doesnt look like what you have
MerCruiser 165 1970-1977 gimbal housing assembly and exhaust elbow parts. Buy a genuine Mercury Quicksilver or aftermarket part.
www.marineengine.com
this is the link to all merc engine and drives
Lookup MerCruiser engine & stern drive by engine model and buy discount parts and accessories from our large online inventory.
www.marineengine.com
I suspect from the holes in yours you have a later transom housing
this parts diagram is from later 1978-1981 165 and #14 is the two orings
MerCruiser 165 1978-1981 gimbal housing assembly parts. Buy a genuine Mercury Quicksilver or aftermarket part.
www.marineengine.com
As Kenny said most likely any place that sells orings like a good napa or a seal and packing place will have them. The Oring dimensions are given in the parts detail if you dont have originals