I was in the same boat a while back. I ended up getting a Zalman CNPS7700-AlCu that didn't use the stock CPU holder. You can try contacting Asus here: Asus Support.
Not sure if they would, but you might as well get an aftermarket chipset cooler. If Asus won't help with the screws, then try a hardware store and bring the mount and backplate with you.