what's the best substitute for coconut butter or coconut milk in a recipe?

What are you trying to make? That may help people know what will work if you give your recipe.

I don't have much experience with it but sometimes, but not always you can just use coconut or other vegetable oil. Otherwise peanut/almond/cashew butter for the coconut butter. Maybe mashed banana or applesauce and a bit of extra oil.

There really isn't any substitute for coconut milk where it is a main ingredient like a thai curry. I'd just make something else. But you might be able to use yogurt, almond milk, soy milk, evaporated milk. There is coconut milk powder that can be reconstituted.