depending on where you live/work, etc is what will determine where you get your supplies. shipping is usually the most expensive, so buying everything at once is the way to go so as not to keep paying the EPA transport fees.
I went thru 15 gallons of resin on my last project (stringers, bulkhead, transom, flotation boxes, motor mounts, coolers). some of it made trophies.... some of it soaked into my clothes, a bit of it got turned into dust by grinding, most of it is in the boat holding the stuff together. I did run-out, and the last 5 gallons purchased locally (as 1 gallon containers) cost me about 3 times the first 10 gallons did.
make sure you get layup resin (non-waxed)