Apple App Store/iTunes
You can find your Apple Transaction receipts by opening the iTunes Store App and scrolling to the bottom and clicking 'Sign In.'

Click 'Use Existing Apple ID,' then enter the Apple ID and password that you use with the iTunes Store. Click 'OK.' When you sign in to the iTunes Store, you also automatically sign in to the Music, App Store, iBooks, and Podcasts apps on your device. For more instructions please follow this link.
Find your order and click on the grey arrow behind the order and you will find your receipt.