Most any supplier will have them. A bonded polyester that is UV protected is common or you can go all out and us a lifetime thread like Tenara or Profilen. A lot depends on salt or freshwater use and what your customer's expectations are.

HI Matt; Thanks a lot; ys I live in the tropics and salt water here are very harsh; I did some clear vinyl curtains using the V-92 polyster bonded but they break after a few weeks.

In addition to UV protection, you'll also want thread that's resistant to mildew, chlorine and bromine. Check out Albright's Supply, which carries a wide range of marine thread.