I've purchased and used a number of fabrics from fabric shops. Shops often have a section of fabrics they call 'lining' or such. I think it was usually rayon before polyester came along.

There used to be a sil-ron sold by Sig or some other supplier back in the 60s-70s. It was significantly easier to handle, took less dope to fill, and was stronger than silk.

I've never used polyspan. Where did you get it originally?