Does a Mesora need a Mekor?

An unwritten Mesorah may be a Rayiah, but not a Mesorah.

You mind giving some more details what you are really getting at?

Every “mesorah” has a mekor (origin, source), only you may not know it. What makes a “mesorah” (tradition, custom, practice), halachically significant is that over time it has become widely accepted. From a halachic perspective, the relevant “fact” is not an origin that might be unknown (and is of interest to anthropologists and social historians), but the current status of its acceptance.

Of course a mesorah needs a mekor. That is why we are so careful to make up a mekor for each of our mesorahs.