You can buy Blackout cloth at many fabric stores, normally it comes in 54" widths, but I think some folks have found it wider, and I think it is like 2-3 bucks/yard. You would have to build a frame to stretch the blackout cloth around, but that is not to expensive either, depending on what wood you use.
I know there have been some people that have used bed sheets, if you do you want to use something with the highest thread count you can find, which won't be cheap, you'll still get some light through it, but it will be better than a cheap sheet, hey that rimes :) But still I would also not recommend using a sheet.
Heck, if you have a wall available in your room, just paint one of the walls with a flat matte white paint, making sure you don't have any texture or orange peel, then you could build a small frame to attach to the wall around the viewing area that is wrapped in velvet, walaaaaa you have a cheap screen.
___________"Don't worry about the future, it is already tomorrow in Australia..."My Website