Charcoal Location - Pokémon (BDSP)

First you need to go to Resort Area.

Click Town Map option.

The Charcoal can be found on route 227.

Select Yes when asked would you like to use rock smash? to break rock.

Select Yes when asked would you like to use another one?

Enter into this building.

Move towards route 226.

Then again come back to route 227.

Select Yes when asked would you like to use rock climb?

You found a piece of charcoal!