It looks like they do carry white. http://www.herculiner.com/questions.html#cI am also going to be resealing the roof of ours in a month or so and am in the process of finding something that is durable and will last.
The website is wrong. I couldn't find anybody selling the white so I called them directly last week and they said they discontinued white a year ago.
Check out a product called Bus-kote, it's also designed to reflect heat.