There are a couple places where you can see a receipt for your purchases which fulfill your orders.
On the Purchase order page
You can find a Purchase Order linked on the Order details page. To find the link to the PO, first navigate to the order details page of an order, and click the name of the Purchase order for any given shipment in an order, and you will be redirected tot he Purchase order page.
Here, on the Purchase Order page, you have all the details of the purchase, including the amount totals and the payment method used, as well as a link to the transaction receipt.
Additionally, you can find the transaction receipt on the Accounting >> Billing history page.