Mark, you can find it at hardware stores. There are lots of different brands. I used to use one called PC7 but haven't been ale to find it recently. I found something similar (don't remember the brand) at Lowes - it comes shaped like a tootsie roll, with both parts of the epoxy in the roll. You just cut off however much you need and kneed it into a putty.
If your repair is really large, you can pt a small screw into the foam of the board to give it something to grip onto.
Once dry, the material is sandable like bondo, and you can perfect the shape. I recommend hitting it with some spray paint to ensure its sealed against the water.
I have used this technique on many boards (mine and friends) and have had great success with it.
[edit] here's a link to the stuff I used to fix my fish -
http://www.lowes.com/lowes/lkn?action=productDetail&productId=191857-133-431348&lpage=none[/edit]
(Message edited by texastbird on September 12, 2009)