I have a receipt for a rented movie but can't find the movie in iTunes or on Video app.

If you rented it on your iPad then it should have downloaded into its Videos app - if it hasn't completed downloading then it might still show in the iTunes app in a Downloads tab in it (Downloads only shows if there are downloads happening), the iTunes app on your iPad is just the store.

If it doesn't show in Videos then does it show via the iPad's spotlight search screen : pull down the centre of the homescreen (not from the very top of the screen, that will open Notification Centre) ?

If you rented it on your computer's iTunes then you will have to connect the iPad to your computer's iTunes and use its Movies tab to move it over to the iPad - rentals are a one-time only download, so won't show on your iPad for redownloading.

If you can't find it on the device/computer that you rented it on then try the 'report a problem' page to contact iTunes Support : http://reportaproblem.apple.com