Cobblestone?

[Adventria]

Where do I find cobblestone so I can finish the last tablet target?

[lilisaur]

Which tablet target is last for you? Its not always the same one for everyone.

And there isn't (in English version anyway) any item named Cobblestone. Do you mean Flagstone? Or Stepping Stones? I'm not sure which tablet target either would relate to though.

[Adventria]

It was the one with all the tile types from the tablet where you make a castle. I found what I was missing though, I did not have enough lava.

[lilisaur]

Yeah that's a tough target. You've got to make 26 types out of just 27 possibilities. It's pretty easy to get to 22 or 23, but the last few require using weird things you might not otherwise use, like Lava/Liquid Lava or Plasma, Bloodstone/Sanguine Sands, and Scales.

This was stumping enough people in the past that I made a guide for it over at GameFAQs:
https://gamefaqs.gamespot.com/switch/215336-dragon-quest-builders-2/map/14945-map-tile-type-guide