I just received a very nice Olympus OM 300mm f4.5 Zuiko.
Unfortunately it does not have the tripod mount.
Were these sold separately? Or is my only hope to find a donor?
When they were new, you might have been able to buy one as a spare part.
Today, you might need to keep an eye for these things.
You can try to contact known Olympus repairers to see if they have that part.
Think they were part of kit of several lens types, ie a common fitting.
Ricardo is correct you could order as spares during production.
There is a parts number catalogue on maintenance database.
But ...