Fossil Pokémon in Pokémon Brilliant Diamond and Brilliant Pearl are the ones you can find as you explore your game. You will only be able to find this item in a specific location, but when you do, you can restore it to run with a Fossil Pokémon. Here's what you need to do to find the armor fossil in Pokémon Brilliant Diamond and Brilliant Pearl.
The only way to find the armor fossil is to dig into the fossil sites of the Grand Underground. This location opens when you receive the Scout Kit in Eternal City from the old man who lives near the Pokémon Center. You will be able to find the excavation sites from the orange dots on the map in the upper left corner. You will then have to take them out by completing the minigame. You will know that you are about to unearth the Armr Fossil from the profile you see under the earth.
Unfortunately, the armor fossil is only available to Pokémon Shining Pearl players. It is not available to owners of Pokémon Brilliant Diamond. If you want to receive the Pokémon it transforms into, Shieldon, you will need to trade a Pokémon Shining Pearl player with it. You'll likely want to bring your fossil Pokémon, Cranidos, to trade it with another player.