1: You can use this one:
http://www3.towerhobbies.com/cgi-bin...?&I=LXBJB5&P=7
2: 3905 diffs will work ok, but you need the diffcases and bearings as well, as they are not the same as the ones on the 3906.
3: I had mine geared 66 on the spur and 25 on the pinion on 4s lipo.
4: Ebay is your best chance.