Heres a good place to get rare settlement resources! Head on over to starlight drive in. Once you are there go up to the top floor look for a red tool box small cabinet and a suit case.
Loot everything in those then go into workshop mode and select the cabinet. After you have selected the cabinet and its glowing hit the cancel button and exit workshop mode. You will see that you can loot everything all over again. The items inside these are all random so keep trying if you are looking for something specific.