I use diamond rotary files and these are available in box sets with loads of other tips to suit a host of rc hobby jobs - great on composites and cheap.
If you want a specific product specifically for coutersinking then the best are from Permagrit - who do a huge range of brilliant products.

marcs

[link]http://www.kentsupplies.com/index.php?route=product/product&product_id=270[/link]

[link]http://www.permagrit.com/index.php?cPath=69[/link]