I'll second the Sunbrella marine grade.
It's also available online (Amazon.com) if there is no retailer near you.
Sounds like you're going to sew your own, so Sunbrella is a good choice. Light and easy to work with. I use to sew mine with a Singer portable and carpet thread. Double over the edge material and iron in nice straight edges with an electric iron, but not too hot. The Sunbrella will hold the crease while you stitch it.
My little Singer worked great until I decided I wanted to use heavier sun resistant thread and sew on borders, bindings and reinforcement material. Upgraded to a Sailrite LSZ-1. Pretty heavy machine, but what a difference in the finished product. (Neat demo video below)
Sailrite Ultrafeed® LS-1 BASIC Walking Foot Sewing Machine - Sailrite