http://www.rodworx.com/accessories.html is a good source. M+ sells the same exact ones for about $85. I have the 15" model on my '66 289, and it works amazingly well. You may need the special radiator cap designed to work with an overflow tank, there has been some discussion about that in the past.
Rodworx will sell you a nice chrome one, too--I think you want the 13lb model.