How can I get a receipt for an iPad purchased over 18 months ago?

If you purchased it at the Apple Store online, then go to the Apple Store online, click your name toward the top right, then click View Account > Print an Invoice.

If you purchased it at a walk-in Apple Store, then if you used Easy Pay, you can print the receipt from the Apple Store app on your iPad. Open that app, go to Accounts > EasyPay Receipts. If you did not use Easy Pay, you should contact the store.

Check your email. Apple typically email's receipts.

Exactly what information are you trying to retrieve? There is probably another way to retrieve it.

Would a receipt from your credit card company be sufficient? That should be easier to obtain.