automatic transmission fluid type A ATF

[Old GI]

While doing my periodic service prior to hitting the road I noticed that the fluid reservoir for the slides (slides in) was a little more than the 1/2 inch below the top of the filler as recommended by the owners manual. I hopped in my trusty F350 and set out for the local auto parts store(s). I came home hours later without the fluid, nobody has that particular type of fluid. Can anyone tell me if it is still made?,or is there a substitute? The level is not dangerously low but I like to keep things filled up (good 1st echelon maintenance, keeps your RV out of depot maintenance), any help will be appreciated. Thanks.