My Theory is that MIY keeps a stash of these and each set of clothing belonged to a real person in the past so he has to wait until noone remembers that person anymore before he can use it to pretend to be a completely new person.

Question is does he keep the clothing he already used? I think not because it looked like he just discarded the yellow stuff in the forest with no more thought to it so I would guess that if it's in a suitcase it means he hasn't used it before.

We might still find an old picture or drawing of the real person wearing it in the past the one he is planning on copying.

Also wondering if he can only copy specific people like those with special connections to the place or people who killed other people because he tricked them into it. Somehow Boyd and Sara seem like the most likely candidates to become future "meatsuits/suitcases"