You can buy a sheet of linen from the local fabric/textile store, it comes in different colors, white or off-white is best. For stripes, paint them yourself or stick a piece of the sheet to a normal sheet of paper with Scotch tape, and just use your inkjet color printer to print them! If you want to make them tougher, you can also soak them in a water7elmer's glue solution and let it dry.
This method is often used for the sails of pirate etc. ships.