How to get planks and sheet metal in Arc Raiders

In Arc Raiders, planks and sheet metal can be bought from Celeste for three seeds each, or 24 seeds for all you need for the Sheltered Retreat project. Planks are common in residential and old world areas like the houses in Old Town in southwest Pendola Pass, while sheet metal is plentiful in technological and industrial areas such as the Assembly and Loading Bay in Stella Montis, where yellow drawer containers seem to be a guaranteed drop.