We do not recommend to purchase any game content in third-party stores as it may lead to account block.
There are 3 different ways of purchasing Eagles:
1. In the official store https://store.gaijin.net/
2. Clicking Eagles icon in the game. In this case you will be redirected to https://store.gaijin.net/ website or to Steam Wallet page respectively.
3. Via Steam. You need to run the game via Steam. You can find out how to import War Thunder to Steam here.