Razor Fang is one of the many evolutionary items that can be found in Pokémon Brilliant Diamond and Shining Pearl. You will need it if you want to evolve specific Pokémon in their final evolutions. Many of these evolutionary items are hidden throughout the game which means you will have to get out of your desire to find them. Here's where you need to go if you want to find Razor Fang in Pokémon Brilliant Diamond and Shining Pearl.
The Razor Fang is available near the Battle Tower. The Battle Tower is a place that you can reach after defeating all the Gym Leaders and reaching the end of the game. It is not necessary to purchase it using Battle Points, which you earn by working through the Battle Tower. Instead, you can find it right outside the Battle Tower, to the left of the massive structure.
If you were looking to use Gliscor, the evolved form of Gligar. You will need to ask a friend to exchange it with you who passed the game. Alternatively, you can have it sent to you by a friend after you grab it and start over. However, you won't find this specific item until you complete the game and unlock the Battle Tower.