If you subscribed via the iOS app then the billing is handled by Apple via iTunes. They will send you a transaction receipt upon purchase. If you can’t find that please contact Apple as we can neither access nor re-send those receipts.
Apple has a knowledgebase article explaining how to see your purchase history and order numbers.
If you subscribed via the web app recently you should have automatically been sent an email with the transactions details (which will be sent every time we process a payment). If you can’t find that, please check your spam or junk folder first. If you still can’t find it or you purchased a little while ago and didn’t receive the automated email, please contact us and we’ll help you out.