Any automotive paint supply store should have it. Its usually $60 a quart. Note; POR 15 does not make a good top coat if exposed to the sun! If used on a exposed surface you will need to top coat it.
A much better coating that has UV but is as strong or stronger than POR-15 comes from www.topsecretcoatings.com
The stuff is amazing and can be had in a ton of colors.