Hi! Some of the snow islands have an ice peak on top of them (there is a random chance to be spawned and I am pretty sure you will recognise it when you will see one). Inside the ice peak there is a cave that contains valuable materials (copper ore, crystal, silicon, rich iron ore).The entrance in the cave is basically a hole in this peak, so just go around it until you find it and then use a jetpack becuse it is above the ground level)
To get the cactus chunk you need to place a "Sap extractor"(can be crafted in the crafting menu) on a cactus. The sap extractor will be red until it is very close to a cactus, and when it becomes green you will be able to place it
Then it produces a cactus chunk every 1:30 minutes and you need to collect them
Hope this will help you:)