Spruce wood comes from a type of tree that can be found in cold biomes in Minecraft. This is a type of wood that can be found at a tall state in podzol or a normal version when in other cold biomes.

To be clear, you only need 7 wooden slabs in total, no matter what wood type of the slabs is. For example, you can use 2 oak slabs, 2 spruce slabs, and 3 birch slabs and it still works for this recipe ...