I just purchased an macbook about 3 weeks ago but I can't find my receipt whwere can I get it?

If you purchased it from the Apple Store online, see the following from https://www.apple.com/shop/help/viewing_changing_orders

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(Opens in a new window) 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.