You can get copies of receipts for anything ordered through the Apple Store in the past 18 months.
Go to the store, clock on Your Account, and log in.
Click View Your Orders.
Pick the appropriate time from the View Your Orders menu on the right.
Click View Order Details on the purchase you are interested in.
At the top of the order detail page, click "Login with your Apple ID to make changes to your order."
Finally, you'll see a "Print all invoices" link at the top of that page. The receipt will be marked as a duplicate.