Is this the mechanical, as opposed to electrical overdrive?
You haven't specified what year, application etc. so I'll make a guess that it's the late 70s early 80s "Toploader Imposter".
If so, it uses the same rebuild kit as a regular (1964-73 or so) toploader 4 speed. Other parts may be available still from Ford, some gears etc. may be available from Jeep aftermarket suppliers as some Jeep vehicles used a version of these Ford type transmissions.