[quote name='bigdaddy']What is the last season? The last one i remember was the one they went into the future.[/QUOTE]
Back to the Sewer, which took place after Fast Forward.
Only Seasons 1, 2, 5 (Ninja Tribunal), and 6 (Fast Forward) got season sets, and all of them except Season 5 were two half-season sets. Seasons 3 and 4 were released in single volumes only. There is a Season 4 DVD, but that is only whatever episodes were not already on DVD.
All are OOP after 4Kids cancelled their contract with FUNimation. The license is now in the hands of Nickelodeon, who released the TV version of the wrap-up movie Turtles Forever, but nothing else from that series.