Go to the indicated coordinates to find the corresponding costume for purchase for Koins.

D'Vorah (Kytinn Queen)
Coordinates: -7, 12 (Chamber of Bones)
Cost: 1,240

Erron Black (Outcast)
Coordinates: -2, 9 (Frozen Graves)
Cost: 1,220

Ferra/Torr (Master and Servant)
Coordinates: 6, 8 (Shadow Spider Kave)
Cost: 1,320 (Timed Chest)

Jacqui Briggs (Boot Camp)
Coordinates: -8, 8 (Garden of Despair)
Cost: 1,900

Johnny Cage (Tournament)
Coordinates: 1, 6 (Shao Kahn's Tomb)
Cost: 1,470

Kano (Tournament)
Coordinates: -15, 19 (Walkway of Souls)
Cost: 2,300

Kenshi (Tournament)
Coordinates: 6, 17 (Dark Pass)
Cost: 1,870

Kitana (Tournament)
Coordinates: 1, 6 (Frozen Graves)
Cost: 980

Kotal Khan (Osh-Tekk Warrior)
Coordinates: -8, 8 (Grotto of Gore)
Cost: 2,100

Liu Kang (Tournament)
Coordinates: 1, 11 (Dark Pass)
Cost: 940

Mileena (Tournament)
Coordinates: -23, 10 (Cemetery Grounds)
Cost: 540 (Timed Chest)

Scorpion (Tournament)
Coordinates: 5, 28 (Spider Gem Hold)
Cost: 210 (Timed Chest)

Raiden (Dark Raiden)
Coordinates: -1, 14 (Dark Pass)
Cost: 10

Raiden (Tournament)
Coordinates: -17, 19 (Walkway of Souls)
Cost: 2,350

Reptile (Tournament)
Coordinates: 15, 16 (Kavern of Doom)
Cost: 470 (Timed Chest)

Additionally, the following costumes are unlocked by completing the indicated steps.

Jax (Revenant): Select two player mode and Kustom Kombat. Play as Jax and select Danger Modifier (1% HP). Win 15 matches (30 rounds).
Johnny Cage (Tournament): Complete the Normal tower.
Kung Jin (Shadow ): Complete the Klassic tower.
Raiden (Future): Log on your WBPlay account.
Sub Zero (Revenant): Complete all 10 Test Your Might stages.