Make your own heatsink. Its cheaper and easy to do.
You have to build your own heatsink. Head out to metal supermarkets (
http://www.metalsupermarkets.com/nonUS/) and pick up a 'slab' of aluminium. Buy some 90 degree 1 x 1 angle aluminium and mount them to the slab. Just drill and tap or just use self tapping screws. Make sure you put a thin layer of dielectric (looks like white grease) in between the 2 pieces.
It'll be cheaper this way as a real heatsink that size will cost you a fortune.
The aluminium is sold by the pound.
or you can buy this from ebay..........
http://cgi.ebay.com/NEW-HEATSINK-ALU...QQcmdZViewItem for $50USD / foot whichincludes shipping charges to Canada.
Good luck