Where do I find materials for production in the frontline workshop?

During the Reign of Chaos events you will be able to occupy specific resource plots, these will then let you harvest the tainted resources, Go to your RoC event screen and the Territory tab to see the territories you own and use the harvest option. The tainted resources you collect can then be process in the Frontline workshops. These produce usable resources and also the Materials that you need to upgrade the honor buildings to higher levels.