I bought a bag of all kind of screws for PC sometime ago (sorry do not remember where). Maybe you need to widen your search, not only hdd screws. The shop I normally use, overclokers click here, overclock click here, thecoolingstore, click here, ebuyer click here, try them.
In the course of building PC's for friends and family I have ended up with a large collection of "assorted screws" (Always wondered where they were supposed to go) Mail me (use the envelope) and I will gladly send you a load.
Also, by the same principle, any local PC shop will have thousands of them lying around. Pop in and ask them for a handful or two - may cost a couple of quid at best and the shop sure wont need them all.
Try your local computer shop. Most shops throw them out because they keep getting more.
I recently popped into a bike shop to buy a hex key set and the guy told me to hang on for a minute and then came out with a bag full of them. They get them with every item they fit and they just bin them.