if my memory doest fail you have to buy it from the mysterious vendor
how do i find them
After some time, when going to Jessica's office she will say she has a message for you, and you will be able to buy the field.With that, you unlock the "FIELD" icon in the map of the outskirts.And the merchant in there, of course.
thank you i have been trying to look of the casing for like 2 days