Where can you find silver powder for Pokemon diamond?

You can't find "silver powder" in Pokemon Diamond. It's not an item in that game. There's a similar item called "Silver Spray" which you can find in the following ways:

* Buy it from the Department Store in Veilstone City for 1,000 Pokédollars.

* Find it randomly in some chests in the Grand Underground.

Silver Spray is a held item that increases the power of special moves by 20%.