Those you found is 320A yep, correct.
You can buy them at any major electronics store.
Examples of online stores is farnell.com or mouser.com
Each ESC has a 3 phase controlled transistor bridge, consisting of 6 groups of transistors.
It might be just 6 transistors, on smaller ESCS. On larger ones, its ususally multiples of 6.
Anyway, what you do is you remove all these stock transistors, and instead you put in one of these, instead of each group.
Of course, you can put 2 or more in parallell of these aswell, to increase max amp to the totally ridicilous.
Allthough its just a matter of 1 to 1 replacement of transistors, i recommend to get some basic understanding of the workings behind it. Find some litterature on 3 phase H-Bridge, to get an overview on how the circuit works.
Its not desperally advanced, just nice to know how its put together before you start working on it.
A most basic introduction can be found here:
http://en.wikipedia.org/wiki/H-bridge
Note the difference between 1 phase and 3 phase bridges. You are looking for the 3 phase one, wich has 6 transistors. Not 4, as the single phase has. Principally, they got a lot in common though, and understanding the single phase first is making it a lot easier to get a grasp on the 3 phase afterwards.