THere are a lot of different sized posts and trying to find one from a catalog seems about impossible. The couple options are; go to the dealer the boat was from, find a place like the others mentioned, or buy a post and base at the same time that match.
I have had luck getting posts from iBoats, Overton's and Cabela's... just need to know exact model
here's link to Attwood's Swivl-Eze line
here's some links to possible help purchasing