Just finished gutting mine as well and I must say the foam is tuff to come by. Best price I found was on ebay Search Urethane Liquid Pour Foam. The guys selling this stuff have a guide on how much you need vs how much your boat weighs. I will try to put a link at the bottom. Hope this helps you out. By the way I would avoid the sheets of polystyrine it is messy, breaks down in weather, is not resistant to gas and other chemicals that will just melt it down rendering it usless.
http://cgi.ebay.com/Urethane-Liquid-...3A1%7C294%3A50