The non-craftable blocks are those who generates mercenaries and enemies. The only way to find some is to explore Castle Defenders’ buildings. You can’t pick them, and if you try to, they’ll break and disappear. So, sorry but you can’t take them home!
Mercenary spawner blocks
They can be found in the mercenary buildings. They generate the mercenaries.
Enemy spawner blocks
They can be found in the dungeons. They generate the enemies.
The enemy knight spawner is a bit particular: it’s not cube-shaped, but it looks more like a redstone torch with a “double-head”.