Trans fluid - can't find type recommended; also rear seal leak stopped on its own?

[centuryhouse]

I searched the forum and read a number of posts here recommending Dextron III, and saying NOT to use Dex/Merc, or Dextron VI as they can cause problems with old seals.

Unfortunately, all I can find at the stores are either ATF, Dex/Merc, or Dextron VI.

Where do you guys get the correct fluid?

Also - what would make my transmission's rear seal leak several quarts after I put my rear drive shaft back on (replaced u-joint & yoke), but then after I moved the car the other day the leak has completely stopped? I drove it again, and still no sign of leak.