How do I find the receipt for a subscription?

The receipt is sent by email to the Apple ID that paid for the subscription. In the case of Family Sharing this can be different from the person who actually initiated ("holds") the purchase.

If you see 'apple.com/bill' on your billing statement - Apple Support

An individual can see and manage their list of subscriptions under their purchase history.

See your purchase history for the App Store, iTunes Store, and more - Apple Support

The receipt is sent to the email of the Apple ID that holds the subscription. Or can be accessed at https://reportaproblem.apple.com for recent payments under the Apple ID that holds the subscription.