I haven't seen anything, but there are numerous references to his early days.
In Heir to the Empire, Thrawn comments on a sculpture made by Ackbar, seeming to imply that Ackbar has some artistic roots (hardly a surprise considering the typical Mon Cal love of anything beautiful).
It's likely that Ackbar was one of the townsfolk "armed with nothing more substantial than farming implements and kitchen utensils" who rose up after the sea ran red.
Ackbar was probably captured then or shortly before (after the Battle, no Imperials were left alive on the surface or in orbit).
In the X-Wing video game, we see the actual mission of Ackbar's rescue, shortly after Yavin.
Presumeably it was after his time as Tarkin's slave that he helped with the refit of Mon Cal passenger cruisers into warships, and designed the B-wing.
"Do fish-people eat fish, or would that be like humans eating monkeys?"
"Humans do eat monkeys. In fact humans eat other humans. . . Y'know, as a species, we are really quite unpleasant."