Where do I find the invoice from the Apple product I have bought?

You should have received it in your email. Or, from Apple:

To view or print your invoice, go to your Order Listing page and select an item to go to the order details. Then, select View Invoice at the top of your order details page to access your invoice.

If you need help viewing or printing your invoice, speak with an Apple Specialist at 1‑800‑MY‑APPLE.

Viewing & Changing Orders - Shopping Help - Apple

The last time I ordered from Apple, the link to the invoice was in a tiny font in the upper right corner of the confirmation emai:

We are not Apple but other end users so can only share our experiences. What I see commonly posted here is that an invoice stays online only for about 12-18 months from purchase. After the yo have to contacat Apple Support.

Search you email. Look for sender donotreply@email.apple.com.