Here are a few links
http://www.gulfstreamcomposites.com/category-s/112.htm (CSC Member's business)
https://www.amazon.com/Pourable-Flot.../dp/B0047R5GK6
2 lb. foam is the standard for flotation. make sure it is closed-cell for marine use. You can call Gulfstream Composits and get more information as well as the quantity needed for your application.
Pour several small batches. If you make too much at once you will have a big unruly blob and end up cutting away the "overgrowth". Be sure to mask off any finished surfaces that may get foam on them; this stuff is sticky and hard to clean up.