Just a passing fan, but couldn't find the answer out on the internet. Was there a Shazam prior to Billy Batson? Did anyone come after Billy Batson, (in comics or tpb)?
NOTE: I don't count the Amalgam character Billy Mar-Vell as a true Shazam.
Yes, there was another Shazam. Sort of.
Back in Ancient Egypt, Teth-Adam was granted powers by the wizard Shazam as the wizard believed him to be pure of heart, however Teth-Adam misused his powers and the wizard had to banish him to the most distant star in the universe. It took him 5000 years to get back to Earth, by which time Billy had been chosen.
Teth-Adam took the name Black Adam, and the rest is history...
Billy was granted powers by the wizard Shazam because the wizard deemed himself too old to use them. So, technically, the wizard was the first Shazam.
From the Captain Marvel Wikipedia page:
Whiz Comics #2 (Feb. 1940) introduces William Joseph "Billy" Batson, a homeless 12- (later 14-) year-old newsboy who sleeps in the subway station of his home city (originally New York City; later referred to in DC publications as Fawcett City). A mysterious man in a green cloak asks Billy to follow him into the subway station. A magic subway car painted in unusual shapes and colors escorts them to an underground throne room, which is inhabited by a very old man with a long beard and a white robe. As the man in green disappears, the old man on the throne explains to Billy that he is the Wizard Shazam, and has used the powers of "the gods" – Solomon, Hercules, Atlas, Zeus, Achilles, and Mercury, hence the name "Shazam" – to fight evil for over 3,000 years. However, he has now grown too old to continue and is in need of a successor. The wizard explains that Billy was chosen due to his misfortune: he had been thrown out by a greedy uncle who stole his inheritance following the deaths of his parents (later retellings of the origin would also note that Billy was chosen for being "pure of heart"). Ordered by the wizard to speak the name "Shazam," Billy is struck by a sudden bolt of lightning and transformed into a superpowered adult in a red costume with gold trim.