It is in Solaceon Town. Go to the cave and at the bottom there it is.
Here are some steps:
If you're in a rush to get to the end, take the top right stairs, bottom left, top right, top left, top left and bottom left. You'll arrive in a room with four item balls: a Nugget, a Mind Plate, Odd Incense as well as HM05 - Defog. -Psypokes
Here is a video to help you.
Hope This Helped :D