you can mail order it from any marine store online like West Marine or look for it at any of the local marine stores in your area.
Applying it depends on what you are applying it to. If it is a repair on your boat, simply get all your shaping done as perfect as possible, and final sand the area with 600 grit. application should be sprayed on for best results. the cheapest wat to spray it is with a sprayer like this:
http://www.westmarine.com/webapp/wcs...01/117/11203/4 . You will need Gelcoat, acetone and MEK (hardener). Mix the gelcoat with 30% acetone, then add the MEK by volume of the gelcoat not the gelcoat-acetone volume. Put it in the sprayer and spray a good heavy coat, but not too much to make it run heavily (small amount of run is ok). Spraying it with this type of sprayer will leave it with an orange peel surface. Wet sand the repair with 1000 - 1200 grit and buff afterward with a good gel coat rubbing compound, then wax.