The driver for all external hard drives is built in to Windows -- it's a Microsoft generic mass storage driver.
As such, there is nowhere to download it from because Windows should already have it by default.
What brand is your external hard drive?
Generally, nowadays, every manufacture has their own official websites to provide many free tools support for his customers. You could check it there.
Or you need this: http://www.computerhope.com/drivers/hard.htm