Just in case you wanted to change out your middle rows of seats, there is an option to do 2 captain chairs. However the captain chairs do not have heated seats and I have heard that they are pretty heavy to move in and out (so they are semi-permanently installed).
For the captain chairs, this is the place that I found sells them:
http://www.hummerfreak.com/index.php?id=68
3rd row seats come in a bench that folds flat into a "bed". Most online H2 stores sell these, some have better pricing then others.
Hope this helps.