Can I obtain Raw Materials without Raiding a Monastery?

You can also acquire Raw Materials by completing the quest arcs of certain zones. When looking at the Alliance Map in your settlement and selecting a zone, the ones that reward resources will show the Raw Materials icon. Early on, Grantebridgescire is one of these - it's one of the two you choose from straight away, to the south of the settlement. I don't remember offhand exactly the amount of Raw Materials earned, but I believe it was enough to build maybe 2 buildings. The reward also gives you a good amount of Supplies as well.

Other zones will show the icon of your settlement, indicating that completing those arcs will increase the progress towards levelling up your settlement (adding 'xp' to your settlement) without actually creating any new buildings. Ledecestershire is one of these.