Doesn't your AutoZone deliver? I know NAPA does as do O'Reilly's and Advanced here in Lincoln. Dex III and Mercon are the same fluid. If the bottle says Dex III/Mercon then you are OK. Now, Dex III is NOT cross compatible with Mercon IV and vice versa. You won't find straight Dex III in a bottle anymore, GM stopped licensing the name and let the patent expire about 20 years ago. They (GM) have come out with new formulations of Dexron since for the newer types of transmissions GM is making, all supposedly backwards compatible with earlier fluids, but as has been said on here before, Dex VI and older trannys that used Dex III aren't happy with each other long term.
You're over thinking yourself into a giant ball of frustration. As long as it meets ATSF and Mil-Spec standards (read the back of the bottle) for the fluid, it doesn't matter whose brand name it is. It will even say on the back of the bottle "Meets Manufacturer's specifications for Dexron III or Mercon fluid when called for" or something to that effect.