After completing Chapter six, you can finally get your first Skell. A level 20 Urban Skell (the only level 20 Skell in the game) is given to you free of charge, and level 30 Skells can be purchased from the Weapons Terminal in Armory Alley in the Administrative District. However, additional Skells can be purchased with credits, or crafted with materials and Miranium. This is a list explaining how to unlock the rest of them.

Skell Frame	How to unlock
Level 50 Skells	Level up Sakuraba Industries to level 5.
Level 60 Skells	Clear the main story.
US0232 Police (level 30)	Complete H.B.'s final affinity mission, "Climbing the Ladder".
US0830 Dozer (level 30)	Complete Bozé's final affinity mission, "Bozé's Ignorance".
US0860 Excavator (level 60)	Complete Alexa's final affinity mission, "Rapid Misfire".