I know the thing you mean and I have seen them referred to as a "cone washer". I've used them on a couple of cars, either to fill in a countersunk hole, or to widen the contact area of a button head screw on a flat surface.
Jet Racing make them, #JE-43 in blue and purple. You can get them through your local Much-More Racing UK dealer.
http://www.muchmoreracing.com/store/...roducts_id=235
http://www.muchmoreracing.com/store/...roducts_id=236
HTH