I've seen a number of special cards such as upgrades, named people and ships and etc in these forums under deck builds and am wondering how/where you get them? Do they only come with specific expansions or can you buy the special cards somewhere like card packs or something?

I'm just starting out in this game

They come in the expansions. There will be no card packs.

The wiki here has the breakdown on the card contents of each expansions-
http://starwars-armada.wikia.com/wiki/Star_Wars:_Armada_Wiki

Every expansion contains cards exclusive to the expansion. So it pays to play both sides in the end.

thank you both very much