What in-game retailer bonuses are available for Final Fantasy XIII-2?

• Head over to GameStop to score an alternate costume for Serah.
• Amazon is offering an exclusive boss battle against Omega in the coliseum. After defeating him, you can enlist Omega as one of your allied monsters.
• Best Buy is giving away the Final Fantasy XIII Episodei novella

Source: Final Fantasy XIII-2 Pre-Order Bonuses Revealed

Edit: Amazon's also offering $20 credit.

The Final Fantasy Wikia had a list of all DLC, including in-game retailer bonuses, but it turns out most of the retailer bonuses were timed exclusives and are available for purchase, at least in North America:

Weapons

• Seraphic Wing^{NA, JP}: PSN/Xbox Live purchase^NA, preorder from Lawsons or HMV
• Genji Bow^NA: PSN/Xbox Live purchase, preorder from Gamestop (if a Power-up Rewards member)
• Calamity/Catastrophe Blade^ALL: PSN/Xbox Live purchase^NA, preorder from Square-Enix online^ALL
• Muramasa^{NA, EU}: PSN/Xbox Live purchase^NA, preorder from Amazon.co.uk^EU, bundled with Nordic edition^EU

Costumes

• Summoner's Garb^ALL: PSN/XBox live purchase^{NA, JP}, Preorder from Gamestop (if a Power-up Rewards member) or EBgames^NA, Preorder from GAME.co.uk or bundled with Nordic edition^EU
• Battle Attire^ALL: PSN/XBox live purchase^{NA, JP}, Preorder from GAME.co.uk or bundled with Nordic edition^EU

Accessories

• Diva Mic and Diva Headphone^JP: Included with "Yakusoku no basho" single
• Goddess' Bangle and Goddess' Earrings^JP: Included with mini-soundtrack
• Stiltzkin's Journal and Durable Stiltzkin's Journal^JP: Included with Famitsu Weekly Issue #1203

Monsters

• Omega^ALL: PSN/XBox live purchase^{NA, JP}, Preorder from Amazon.com^NA, Preorder from GAME.co.uk or bundled with Nordic edition^EU

Notes

^ALL: Available in all regions
^EU: Available for the European/PAL region
^JP: Available for the Japanese/Asian region
^NA: Available for the North American region