PC: Unlocked by completing the last map with an "A" or "S" rank, getting an "A" or "S" in Mining the Depths will unlock Rail Yard, getting an "A" or "S" in Rail Yard will unlock High Seas Fortress, and so on.
PS4, Xbox One, Nintendo Switch: All maps are unlocked by default.
Note: In the PS3/PS4, Xbox 360/Xbox One and PC versions, these costumes cannot be used in the story mode. However, the Nintendo Switch version allows all EX costumes to be used in the story.
How to unlock
All EX1 Costumes are unlocked by playing with the DEFAULT costume of a character in The Mercenaries or The Mercenaries No Mercy and obtaining an "A" or "S" on either SOLO or DUO mode in every version of the game.
For example, getting an "A" rank in any map as Leon Scott Kennedy (DEFAULT) will unlock Leon's EX1 Costume. It will not unlock Leon's EX2, EX3, or another character's EX1.