Timothy:
I'm not sure why Season 6 went out of print. It was originally a MOD DVD-R release from Universal which they then quietly switched to pressed disc release.
For Region1, these are all long shots at best but here goes....
The Defenders Seasons 2 through 4
East Side/West Side (all 26 episodes)
'Way Out (all 14 episodes)
The Nurses aka The Doctors and the Nurses for seasons 2 and 3 (all three seasons)
Bus Stop (26 total episodes)
Cliffhangers...