The razor claw is only found as held item by wild Pokemon. It is a 5% chance a Jangmo-o or Hakamo-o are holding it, and a 50% chance a Kommo-o is holding it. If you want to get a razor claw yourself, I recommend either catching every Jangmo-o you find, using the move thief on every Jangmo-o you find, or SOS chaining for a Kommo-o catch it and hope it has the razor claw.
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