Go to your account, there is an option in the middle of that to show previous purchases. The receipt is there. As an example, the receipt will look like this.
I assume this is using the desktop iTunes app then? I don't see an account option on the iPad/iPhone app store.
I'm having this issue. Bought the subscription twice (once by accident, I just wanted to try the game out). I'll go ahead and PM tsagoth, I'd greatly appreciate it if I get this resolved.