Here is an example search on counter sink washers at AmainHobbies:
http://www.amainhobbies.com/advanced...ersink&x=0&y=0
An alternative is if you want to use cap head screws you might be looking for these kind of washers:
http://www.amainhobbies.com/product_...-Washer-Blue-6
Just examples. Ebay is probably a better source.
Cheers