I was able to find EPS at Maui Home Depot. The local surfboard fabrication supplier Maui Fiberglass had foam, but it cost twice as much.
I got fiberglass cloth, epoxy, and more at Fiberglass Hawaii, our local surboard fabrication material supplier. The not only provided materials that had relevant advice. "The bigger your skeg is the straighter you go. The farther back your skeg is the staighter you go."
REI has lots of kayak fitting stuff, shock cord, nylon webbing, buckles, etc.
The EPS from Home Depot broke before I got it home. It was windy and I wasn't skilled at moving around 4x8 sheets of fragil EPS. When the sheets snapped I was holding them in the middle trying to get them onto the roof of a Honda Element. The broken sheets fit inside. In retrospect, I should have held the EPS sheet by the upwind end and let the other end blow in the wind. Later I bought a third sheet, cut it into 2x8 pieces and it fit inside our Honda CRV.
The break was pretty clean, but not straight. I decided to glue it back together with thickened epoxy.
I stacked the two sheets, keeping them separated with wax paper. The epoxy was thichened with Cabosil and portland cement. The joint turned out much stronger, stiffer and hard to cut than the EPS it was holding together.
I held the EPS sheets in place with containers at hand. My two sheets of EPS had cost $35, later my wife's cousin gave me some styrofoam packaging from his new Costco furniture. If you're into recycling and have time on your hands, you could probably glue blocks of salvage polystyrene to build a kayak. My experience is that polyurethane glue is stronger than the EPS it bonds.
I used Great Stuff foam to fill large voids.
Because I had Great Stuff on hand, I filled the bow hold. It seemed to add a lot of weight.