Raw Materials are obtainable only by raiding monasteries. You need the help of your crew to force open doors and lift the ornate lids off of chests, so to grab the Raw Materials, you’ll need to raid the area.
There is only one monastery in East Anglia where you can find Raw Materials. You’ll be able to acquire 3 lots of Raw Materials from different chests in the monastery, for a total of +60 Raw Materials. You will also be able to obtain Supplies from the chests, which can also be used to construct buildings in your settlement.