Don't forget you can paint your own as well.
The great ones have no wrinkles and better reflection. Your white sheet is not a bad place to start as you can figure out how big a screen you need. And there is paint. Link follows.
http://www.amazon.com/Carls-Blackout-Projector-Material-66x110-inch/dp/B007KA07YM/ is #1 at amazon. Ever wonder why?