Katherine,

The other "standard" source for un-heat-treated polyester is:
George Dyson: gdyson@gmail.com
George has lots of different fabrics including polyester and nylon. He will send a description and price list.
His business practice is a little different.
You send him an order, confirm it, and he sends the cloth.
You then send him payment, snail mail.

If you want really lite cloth you can also get it at Aircraft Spruce and Specialty. This is primarily used for aircraft, where they don't have to bump into logs, rocks, and the beach.
Most people use something around 8-9 oz.

What boat are you going to build?

Oh by the way, Welcome to the forum, you are fairly unique, to my knowledge.
You should have fun with the build. Lots of help here and different ways to do the builds.

How are your wood working skills? Do you have the tools?

Good luck.