You can also just buy them in some shop. They are kinda expensive, but gold should be so plentiful by the time you are marrying girls. Tax should be enough, but you can also use Sandra for a while and rob stuff in combat. It gives you 20k or 50k pretty regularly. You can also do the fire raid. You have to run this a lot anyways, and the yellow chest gives a lot of gold.There is a trick, if you exit the area with any resource in it and then enter it again, they will respawn.
Same happens if you enter skilltree and then exit it. This way you can cut down running around to 0.
So, stand near the node, mine it, then enter skilltree, mine again, ad infinitum.
Honestly though, at some point, just editing the save file to skip gathering the same materials over and over is an option. It takes some discipline to not abuse it though. Towards the end game, I had no issues editing materials when I find that I am just sitting there running through the mines repeatedly. In the end, it's a single player game and the point is to have fun. Grinding for the end game equips from raids is gonna be quite satisfying and fun. Running back and fourth to gather a huge amount of a common material is gonna be a mindless and boring task for most.