There is a list of sites near the top of this bread that can be accessed free.
Military flights a difficult to track live yet alone trying to find old flights.
If they don't want to be tracked for security reasons then there is little that we can do about it.
Often they will drop off the tracking systems without warning.
Combat planes like fighters and special operations flights rarely show up unlesss it is a training flight.
To find any data would need more info to begin with, aircraft type, callsign, tail#, departure/destination airfield etc.
Without more data it is like looking for a needle in a haystack.