If you bought Final Draft 10 / 9 / 8 as a download, you can always download the full installer for free from our website. The installers are HERE.
Once you've downloaded the installer, you may want to copy it to a USB drive, DVD/R/W or CD/R/W or some other storage device for future use. If you don't want to keep a copy of the installer, simply download it again when/if you need it. There's no limit to the number of times you can download it.