In Pokémon Shining Diamond and Shining Pearl there are several Pokémon that have regular evolution items. One of those evolutionary items is Razor Claw. You can find it during your journey in the game and you can locate it in a specific location. So, here's where you need to go to find Razor Claw in Pokémon Brilliant Diamond and Brilliant Pearl.
You will need to get to the end of the game if you want to find Razor Claw. You can find it on Victory Road, the area you'll need to reach if you make it to the Pokémon Lead and challenge the Elite Four. Along the way, you'll find Razor Claw on your way to the Elite Four.
When you have the item, you will be ready to give it to Sneasel, who can use it to hold it and to evolve into Weavile. You can only do this by having Sneasel hold the item and then level up while it is night. You have to wait for game time to turn into night and darkness to come. If you don't, Weavile won't become Sneasel, but it will keep Razor Claw until it goes through evolution. However, you have to wait until the end of the game to receive Razor Claw.