You can try to make some yourself. Polycast, the material the windows are made from, is relatively cheap. Blowing the window is not actually that technically challenging, plenty of reference on the web on how to. Then you just need to get an IA to see if he can get a Block 3 approval for a one-shot 337. I really wouldn't do the rear windows with too much dome as you may start to get some turbulence on the rudder.
THe other route is to go to the http://www.skywagons.org/
and buy the STC list. Look to see if any has ever been done, if it is no longer available you can gain approval as 'previously approved data.'http://sites.google.com/site/thesubmarinesofjoseberrios/build/build-plexiglas-observation-bubbles/build-the-bubble-form