ATF A is still available in most of Europe, and has been replaced in the US by Dexron
or ATF - A Description - Kroon-Oil Gear Oils Automotive - Catalogue - Kroon-Oil
I found that I had some old original Dexron (no numbers or suffixes) and I used that in my Kanzaki gear box that was sticking in gear. It fixed it after two changes and some hours on the box.
I am told by the Shell Lubricants people that Dexron III is the replacement for ATF-A and that you can find the data sheets on line to compare specs.
The newer Dexron's (II, VI, Saturn, etc) are NOT the same and I can not suggest using them.
You may be able to find ATF-A at a "farm" or lawn mower shop as it is still used in some AG/Offroad applications.