Here's a link to many organic clothing suppliers:

www.ecomall.com/biz/clothing.htm

Ask for samples. Most manufacturers have no problem mailing you a scrap about the size of a dollar bill.

Put them through the 3 second test:
Stand over a sink with a little water in it.
Hold the fabric sample so that it hangs out and down.
Take a bic lighter (natural flame, not the "turbo" types)
Light the lighter and use it to push the fabric back a little bit so that the flame is partially under the fabric (but mostly to the side).
Count 3 mississippi.
The possible results are:
- Fabric flames out and is destroyed: fail.
- Fabric ignites, gets a hole in it but goes out on it's own: flame resistant - acceptable but not for drapey things.
- Fabric hardens and is ruined but does not ignite: flame resistant - pass, though not recommended of area where contact with fire is known to happen (body wraps and such.
- Fabric warms but does not ignite and is usable afterwards: fire resistant - pass, for all uses.

Record your information (fabric type, maker, test results, etc) and post it here in the Fire Arts FAQ tribe, and/or send to NAFAA, they love that kind of stuff.