You want to do your research before/if you open your first can of resin. Watch videos of people using it (sorry, too lazy to pick out the better videos...I really should do that some time). read plenty of tutorials. Cut out a random chunk of foam about the size of your hand and try to go through the entire process. of prepping, fiberglassing, surfacing and painting it.
Adding to what nathancarter said, last time I purchased insulation foam, I just had a swiss army knife on hand and scored and split the thing in half in the parking lot. So if you gotta work to find someone competent in the store, you might not want to sweat getting them to do it.
General-purpose fiberglass & resin can be purchased from most large hardware stores. It's usually near the adhesives section. More exotic products like colloidal silica or gelcoat often need to be purchased from marine supply stores or ordered online. For online orders, tapplastics.com is pretty good for fiberglassing supplies.