How do i receive an invoice or download an invoice, just purchased new iphone?

See the following from Viewing & Changing Orders - Shopping Help - Apple

Once an item is shipped or picked up, Apple will create an invoice for your order. 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.

If this was an online purchase, you should have gotten an email receipt. If not, use the contact support link at the top of the page to contact Apple directly.