Does anyone have the specs/size of the screws that come with the PRT-15773 Aluminum Heatsink Case?
Don’t have one to check but looking at the picture my guess would be a M2.5 or M3 sized screw.
Might take it down to your local Ace hardware (or other local hardware store) and see what fits. Small local hardware stores usually have bins of screws that are loose so you can try them out.