I couldn’t verify a clear answer from the available info, but the closest clue says Reinforced steel spawns in Earth Island Crafter in Singing Monsters 3D. The same source also mentions Rusty Coil in Eternal Ruins, Glowfern in Plant, and Activated Shard in every island except Cold.

What that likely means

  • If you’re asking where to find Reinforced , it appears to be tied to Earth Island in the crafting area.
  • The wording in the source is not perfectly clear, so this is the best-supported reading rather than a guaranteed full guide.

Related item hints

  • Rusty Coil: Eternal Ruins.
  • Glowfern: Plant Island.
  • Activated Shard: every island except Cold.

Sorry, failed to confirm more detail from the available source.