unfortunately, you can not buy the "right" screws at a hardware store. The screws Graco uses are screws designed for the use in plastics. There are some significant differences between the screws provided from graco and the hardware store screws, but to the regular person, you wouldn't really notice. There are differences in the thread angle, the diameter of the shaft, the depth of the thread, possible even screw material - these details make the graco/plastic screw perform better with the material in the booster seat.
I'm not saying that the hardware store screw is better or worse, just that it was not designed and tested for use in the turbo- the plastics screw was. And since Graco requires screws in the armrests, sounds like these little screws are somewhat important.
We talk alot about respecting the testing that is done and not adding or subtracting from what the manufacturer tests to - mostly in the aftermarket product regime, so while we want to give information about possible solutions, I think the manufacturer needs to approve the use of hardware store machine screws as an equivalent and appropriate substitution.
Now if Graco CS is telling parents to go out and buy a machine screw, as mentioned in PP, I would hope that an engineer has approved this substitution, and given a clear and identifiable part number/specification. I would also hope that if Graco is approving an off the shelf machine screw, they will put it in the turbo manuals and notify customers of this new information.