You can download many of your previous iTunes Store purchases through iTunes in the Cloud. You may be able to download this particular purchase using this feature.
To learn what previously purchased content is available for download in your country, visit this article:
iTunes in the Cloud and iTunes Match Availability
Some items you’ve previously purchased may not be available for download. For some movies, you will see the message "When purchased, this movie will not be available from iTunes in the Cloud” if the movie can't be downloaded again. This message appears under the Buy button on the iTunes Store product page.
You will need the latest version of iTunes. It’s available on the Apple website:
Installing the latest version of iTunes will not affect your library or any items in your account.
To learn how to download past purchases, visit this article:
Downloading past purchases from the App Store, iBookstore, and iTunes Store
Note: If you are downloading items again to replace files you have on your computer or device, be sure to delete the original files first.
If you don't see what you want to download in the list, it may have been removed or modified since you purchased it and is no longer available.
If you lose your purchases, you can restore the items from your backups. This article explains how to back up your library:
iTunes: Back up your iTunes library by copying to an external hard drive
If you're using a Mac, you can back up your library automatically with Time Machine. This article will show you how:
Mac 101: Time Machine