I wanted a 10GB max hard drive for an old computer and eventually managed to get hold of an 8GB second-hand one.
You may be able to get hold of one from a local computer shop, which does repairs and upgrades. That's where I got mine and was given a 3 month warranty with it too! It's still running after 6 months without problems.
If you buy privately, you may find, like I did, that the hard drive may fail very quickly (10 minutes to be precise) - in my case, the person I purchased it from refused to refund me the money. A case of buyer beware.