Computer fairs always seem a bit pricey to me except for media. PC World are convenient but expensive.
Try
www.savastore.com. Prices are low, and they have a huge range, although after sales service is said to be poor. I bought a Lacie 250GB ext HDD from them and it works fine. The one big disadvantage is that I have yet another power supply to plug in, as the Lacie needs external power. It comes with basic backup software, that allows me to select which folders to save.
As someone else suggested, you could buy a HD caddy and insert it in one of the optical drive bays (removing a CD/DVD player if need be). Then you just buy internal hard drives to fit the caddy. In the long run this is cheaper. But you will need to buy separate backup software. There is some free stuff available, and it works, but is not especially elegant.