Most additional costumes for characters are unlocked via the Side Missions that are available after you beat the campaign. You need to beat the side mission once, then play it again in Free Play (so you can use characters with the right powers to access everything).
Within each side mission you can unlock:
- One additional costume - A Red Brick - 3 Gold bricks (completing the mission, rescuing Stan Lee, and getting True Believer) - Whatever heroes/villains are unlocked from simply completing the mission once
My Gaming Blog: http://thesaveroom.blogspot.com/ 3DS: 2320-6678-6610; PSN: Solid2TheSnake
Try going to the character select screen at a SHIELD terminal and waiting for the costume/character to rotate and become selectable. If you can buy it, just click X at the time it pops up and it'll be there.
Hoist the colours high! When I get sad, I stop being sad and be awesome again. True story.