Wow.. not a single reply.. :|
The screws that hold the hard drive are standard screws for that type of drive. Any computer store should have them. To be honest however, I just "borrowed" two screws from one drive and used them on my add-on drive. I used them on opposing corners. I've had it like this for over a year w. zero problems.
As for the outer screws, I'm not sure if you can put flush mounted screws without mod'ing your case. If you simply want flat screws, remove one of them and take it into a good hardware store.. Sorry, I don't know the size...