How can I get my iPad receipt?

That would largely depend upon where, when (or how) you purchased your iPad…

If purchased directly from the Apple website, or Apple Store App, you should receive a confirmation email - and after your product ships, a further email with your invoice should be sent to you. Otherwise, your receipt may be available from within your AppleID account. Install the Apple Store App - access the For You tab, then tap your initials at top-right of the screen. Next select Your Orders; if your purchase is visible, you’ll find a link to the associated invoice.

If purchased from a retailer other than Apple, you’ll need to approach the retailer from which your made your purchase.