I had one run, where I was constantly out of coal, but in my last few runs, there have been most of the time more coal than I need, even with fish smokers. There are coal nodes both in Skull Cavern and Volcano, and dust sprites drop it. In early game it's sometimes scarce (and buying it or using charcoal kiln are not good options as then there isn't a lot of money either. Actually in my last runs the scarce resource seems to be -- wood. Normal wood. OK I've not been crafting tree fertilizer, I'm waiting for the bone mill...