Believe it or not, the home Depot near my house had a gallon can of the resin, complete with two tubes of catalyst. I believe it was Bondo branded, even cures in two hours! Works great for me so far.
They even the fiberglass mat, but only in small sizes.
If you live near the water, check out a boat supply store. I live near Annapolis MD, and the boat store I went to had everything, it was like a candy store of Fiberglass materials.
Look in your yellow pages under fiberglass REPAIR. Many of the places that do this kind of work, will have gelcoat. Call them to see if they will sell some to you. Not all will, but you only need to find one that is willing. They will probably require that you bring your own container. Empty plastic cat litter jugs work very well for this. Good luck. 8)