Well, since Black Adam will become more of a household name, or at least known more to the casual movie goer in a relatively short time here is a list of the bigger Black Adam key issues.
Marvel Family #1: The Big one! Black Adam’s very first appearance. Very pricey and very old comic!
Shazam vol 1 #8: Reprints Black Adam’s first appearance from Marvel Family #1
Shazam vol 1 #28: Another big one! Black Adam’s first “modern” appearance, 2nd overall and Very First DC appearance after they acquired the rights from Fawcett.
All New Collector’s Edition C-58: 2nd Modern/DC Black Adam. Oversized (Treasury Edition)
DC Comics Presents #49: 3rd Modern/DC Black Adam.
Black Adam The Dark Age #1 1:10 variant (Just a classic Ross cover)
I’m sure there are more but these are probably the most popular or at least a very good starting point especially if you have the cash for Marvel Family #1