If I'm not mistaken, you get copper from the mining spot near the bridge. You just need a lot of strength to get it, because you get better rewards the higher your stats are. If you want it for the electrical stuff, you can buy already finished parts from Roman I think.
As for the onions, it is possible to run out of some resourcess. I will edit the potato quest garden and put more plant types in there, so that you can always get atleast one free plant to start your own garden again if you run out.