How can I get a receipt for an old device purchased in 2019?

If purchased directly from Apple's online store, the device will be associated with your AppleID account. As such, you may still be able to download a copy of the original invoice...

On your iPad or other Apple device, install the Apple Store App from the App Store:

https://apps.apple.com/app/apple-store/id375380948

Once installed, open the App - then select For You from the sidebar; this will open a list of devices associated with your account. Tap the name of the device in which you are interested - and if available, you will see a See order details link. Follow the link to find the associated invoice (if available for download).

Only device purchases associated with your AppleID account will show any order details.

Then the likelihood of being able to obtain a duplicate receipt for your purchase will be vanishingly remote...

Given that Apple will only accept device returns for exchange or refund within 14-days of purchase, unless I am missing something here, the only need to produce a receipt for a purchase made five years ago will be to prove ownership of a device to Apple.