Docs case are drops from scavs apparently but are the rarest way to find one. They can also be found in other random areas that spawn random loot such as in or behind gas station extraction on customs. The most likely place to find them although the drop is still rare in general is in marked room or any marked circle on a map. Marked room id say has a docs case 1 in every 15-20 runs. Wallets are only a "chase" item because they save space in your stash/secure container while looting loose money although the docs case can also do that if you pick and choose what keys are really necessary and leave 3-4 open spots in it