Rename findAPortInUse and findAPortNotInUse.
The new names should be unambiguous and easy to type and follow the same tense/grammar. The name should not use 'open' or 'closed', as something like findAnOpenPort could imply an available port or a port with status 'open' (in use).
findAnAvailablePort (findAFreePort seems ambiguous)
I wrote this issue a while ago. There's enough people using portscanner now, that changing the API for a minor name change does not seem worth it.