I can't think of a way to get you cases when you are level 12. You cannot bring any case except ammo into raid so you'd have to barter from a trader or buy off flea, which neither you can do at level 12. I think you can barter for a medicine case at level 13 via therapist. Otherwise you need to grind level 15. I offer a leveling service to level 15 if you want to PM me. I've done a couple people's accounts I can give you their info to confirm reference too.
The other way to get cases is to upgrade your hideout and craft them, which also will requiring leveling up levels and traders.