in the national park talk to a girl with a Persian at the entrance far right
That's the Quick Claw!
The Razor Claw can be found in the Battle Frontier shop beside Olivine. I don't know any other place it can be found.
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You get it as a battle dome item after you get enough battle points.
Yes, you will be able to buy a Razor Claw in exchange for Battle Points but in Pokémon HeartGold and SoulSilver, you need 48 points in order to buy it.
you can by one with BPyou get it in the pokeathalon dome sometimes and it can be held by wild sneasel or rhyhorn.
You can buy a razor claw from the PokeShop, if you have enough Pokemon Coins!
Gligar evolves when holding a Razor Claw at night, I believe.