The mounting screws on the hard drive are usually a T6 (sometimes a T8, but I think that's only the older models). If you have good electronics store nearby (not a Best Buy or something that like, but a Fry's or a real electronics store), they'll probably have one, or you can order it online.
Ah, somehow I completely forgot the HDD mounts with Torx screws. Home Depot sells a Husky tool with several different torx tips in it, it's actually pretty high-quality and it's only $6. I have this, and it works very well.