Where do I find monster carapace?

Similar to my answer here, talking about Monster Hide, you can dismantle materials such as Wyvern Hide into Monster Carapace.

According to other sources, you can also purchase it at high level armorsmiths, though I have not seen that.

You can convert these items into monster carapace:

• Basilisk Hide
• Forktail Hide
• Wyvern Hide