First of, use the proper size screw driver. #1 reason for stripped screw heads. Not sure why you would have to re-paste a GPU more than once unless constantly tearing card down for whatever reason. A single application of quality TIM should be sufficient for the life of most any GPU. Unless of course if playing with LN2.
 
Contact EVGA support, they may be able to get you new ones. If you have an old similar card, try using the screws from it. WARNING: Screws/springs from a different card may fit and screw in, but may not apply the proper pressure. Temps will be the best indicator.
 
Someone from the forum may have put a waterblock on their same card and no longer need the screws. Maybe mention what card you have. I have spare spring screws for EVGA's GTX580 and GTX680+ if the same.