One thing I always do to get cores is as soon as I am able to craft a karve I go to the Ash Lands. Take food with you and plenty arrows that's a good place to harvest cores and coles as well. Take graydrarf eyes regular wood and enough fine wood with you on your trip so you can when needed repair the carve and place a portal in the Ash lands so you can after your trip hop back and forth when needed. Tip for the Ash lands is when you got low gear to stay wet because if you get hit by a surtling the fire weares off after a second or two and won't hurt much. If you already have enough items to build one portal place it in your home before going on the trip so you don't have to make the long way back home in the karve.