Go to one of those fabric shops that sell end of rolls etc.
You'll find heaps of different materials and fabrics that you can use.
Most softboxes etc use a nylon fabric, and these can also be found at these fabric stores.
The best thing, the stuff they have is cheap as they buy fabrics from clothing manufacturers etc.
Might even be worth your while to buy some see-through coloured fabrics too as these can give some good effects.
I don't know if you needed specifically the satin stuff, but if all you need is to build your own light modifier, go to Jo-Ann Fabrics and buy some ripstop nylon. Look around though, because they usually have multiple similar fabrics and you want one that seems pretty white and pretty translucent. Don't go for too translucent though, cause it won't diffuse the light. You want to see your fingers through, in store light, but not the texture of your skin.
I built a 5'x3' diffuser with this stuff and I added a layer of "soil separator" from Home Depot between the nylon and the flash for extra diffusion.http://www.homedepot.com/p/Soil-Separator-Trench-Wrap-36150SSF-6/100154781
This is a very thin, mesh like fabric that blocks very little light, but it's a pain in the butt to work with because it's very fragile. I only used it because I had it laying around from a yard drainage project.