Listed below is how to unlock all of the existing alternate costumes in Injustice: Gods Among Us. Creative Director Ed Boon has mentioned there will be 100 costumes in total, with some due in a future patch.
The Regime and Insurgency costumes are bought through the Archive. These are the costumes you see throughout the story mode – so the ‘unmasked’ Deathstroke, Killer Frost with a mohawk, etc.
Some costumes are unlocked via the Injustice game on iOS. This obviously means you have to play that to unlock these costumes (as it ties in with your Warner Bros ID) but if you can’t or don’t want to play it, you can have someone else enter their ID to permanently unlock the costumes on your save.
Some costumes were only available by pre-ordering through certain retail outlets or by voting in the Battle Arena promotion. These are obviously impossible to get now, so we presume they will be made available in a patch further down the line (as tends to happen with pre-order bonuses).
Regime Aquaman: Unlocked through the Archive
Flashpoint Aquaman: Obtained by purchasing the Flashpoint Pack or Season Pass
Regime Ares: Unlocked through the Archive
Knight Fall Bane: Unlocked by downloading and rating the Injustice on iOS
Regime Bane: Unlocked through the Archive
Arkham City Batman: Obtained by pre-ordering Injustice from Zavvi (UK), Walmart (US)
Batman Beyond: Unlocked by purchasing a Batman card for the iOS version of the game
Blackest Night Batman: Obtained by pre-ordering Injustice from ShopTo.Net (UK), Best Buy (US)
Insurgency Batman: Unlocked through the Archive
New 52 Batman: Available with the Collector’s Edition
Regime Black Adam: Unlocked through the Archive
Arkham City Catwoman: Obtained by pre-ordering Injustice from Zavvi (UK), Walmart (US)
Regime Catwoman: Unlocked through the Archive
Regime Cyborg: Unlocked through the Archive
Flashpoint Deathstroke: Unlocked by buying the Flashpoint Pack or Season Pass
Insurgency Deathstroke: Unlocked through the Archive
Regime Doomsday: Unlocked through the Archive
Elseworld’s Finest Flash: Unlocked by getting three stars on all S.T.A.R. Lab Missions
Regime Flash: Unlocked through the Archive
New 52 Flash Skin: Unlocked by completing Speed Run in Battles mode
Arrow: Obtained by being one of the first 5,000 people to vote in the Battle Arena promotion. This costume also has a different voice actor – Stephen Amell from the Arrow TV show.
Insurgency Green Arrow: Unlocked through the Archive
New 52 Green Lantern: Unlocked by finishing a bonus battle in Injustice iOS
Regime Green Lantern: Unlocked through the Archive
Yellow Lantern: Unlocked by winning one Ranked Match
Arkham Harley Quinn: Unlocked by purchasing a Joker card on Injustice iOS
Insurgency Harley Quinn: Unlocked through the Archive
Regime Hawkgirl: Unlocked through the Archive
Arkham City Joker: Obtained by pre-ordering Injustice from Zavvi (UK), Walmart (US)
Insurgency Joker: Unlocked through the Archive
Regime Killer Frost: Unlocked through the Archive
Insurgency Lex Luthor: Unlocked through the Archive
Kryptonite Lex Luthor: Unlocked by completing all S.T.A.R. Lab missions (you don’t need three star ratings)
New 52 Nightwing: Obtained by reaching Level 30
Regime Nightwing: Unlocked through the Archive
Regime Raven: Unlocked through the Archive
New 52 Shazam: Unlocked by completing all S.T.A.R. Lab Missions for Shazam
Regime Shazam: Unlocked through the Archive
Regime Sinestro: Unlocked through the Archive
Boss Grundy: Complete a Classic Battle tournament on any difficulty and with any character
Red Son Solomon Grundy: Obtained by pre-ordering Injustice from GAME (UK), GameStop (US)
Regime Solomon Grundy: Unlocked through the Archive
Godfall Superman: Complete the game in Story Mode
New 52 Superman: Available with the Collector’s Edition
Prison Superman: Unlocked by beating all battles in the iOS version of the game
Red Son Superman: Obtained by pre-ordering Injustice from GAME (UK), GameStop (US)
Regime Superman: Unlocked through the Archive
New 52 Wonder Woman: Available with the Collector’s Edition
Flashpoint Wonder Woman: Unlocked by buying the Flashpoint Pack or Season Pass
Red Son Wonder Woman Costume: Obtained by pre-ordering Injustice from GAME (UK), GameStop (US)
Regime Wonder Woman Costume: Unlocked through the Archive
Pillars of Eternity the White March Side Quests, Tasks, and Companion Guide
Is More Of The Same A Bad Thing? Not For Infinity Blade III
How to Unlock Outlaw Missions in Need for Speed 2015 Guide
Diablic Scorecard - Why Some Games are Immune to the Metacritic Blackhole
Rita Oras cleavage upsets UK TV viewers