The only way to purchase Gems is through the in-game shop (processed securely by the iTunes or Google).
There are many third party websites offering in-game currency for Clash Royale, but NONE of them are safe to use. Ignore whatever their websites claim, they are NOT sponsored by Supercell. Often, third parties will ask for Game Center/Google login credentials. This will give them access to your game. If you give them those credentials, we cannot guarantee your account's security.
If you purchased Gems from somewhere other than the game, we unfortunately cannot assist you any further, as your account has already breached our Terms of Service.
We reserve the right to ban game accounts with Gem orders from third-party websites.