How can I get outfits?

Apart from the preorder bonuses or the purchase of the Nordic edition of the game, there is currently only one free outfit: the Style and Steel outfit for Serah. It can be downloaded from the PSN Store or Xbox Live just like any other DLC and from the game (DLC on main menu).

Outside of that, outfits—like the Coliseum—are purely for paid DLC.

The current outfits for sale are:

Serah Summoner's Garb (Part of the Omega DLC/Pre Order bonus), Beachwear, N7 Amour,
Style And Steel and White Mage Attire

Noel Battle Attire (Part of the Omega DLC/Pre Order Bonus), SpaceTime Guardian, N7 Armour, Ezio Auditore and Black Mage Attire

(accurate as of 18th May, subject to edits in future)