http://www.amainhobbies.com/product_...-Head-Screw-12
It's a 2.6mm thread.
Best price I've seen, and you get several screws. The hex head is for a 2mm wrench so you can get it tighter than those horrible phillips screws, just a dab of thread lock and your set...I recommend using a washer under it if you use plastic servo horns just because.....