A drama source for fire-retardant fabric...
This is the theater source we used. They have more varieties of muslin than you can shake a stick at and all come either FR (fire resistant) or, regular with the option of treating the fabric yourself (look at the menu in the far left corner.)
Rose Brand
Peace,
Lisa
CreativeCarol also notes... We used Rose Brand to purchase drapery fabric for our small stage. They will send samples so that you can pick the correct color. We bought a fire retardant fabric that was a velour. Too heavy for a tent. But they are a great source.
Be sure to ask for the certificate when you order and save this to show the Fire Marshall! Here is a photo of our stage curtains: