The Venator-class Star Destroyers were pretty much a dedicated carrier platform, but was largely abandoned by the Empire in favour of the Victory and Imperial-class. According to the fluff, Venators carried 192 Starfighters, vs. the ISD's 72. Also according to the fluff, by the time of Yavin, Venators could also be found in the hands of many different factions including some criminal enterprises and even the Rebellion itself.
Of course, since they didn't appear until Episode 3 and the Clone Wars, their presence in the Alliance fleet would need to be Retconned...
Edited by Agatheron