There are many options for projection material. Scrim is a woven fabric that can be front or rear projected. When the scene behind it is lit, the fabric appears translucent or even transparent, but the projection will still be visible. It comes in a variety of widths and is an excellent professional looking material.
A less expensive option is tulle, which can be found at your local fabric store. It also can be front or rear projected, but has a more open weave than scrim. It also is not available in large widths. It comes in a large variety of colors. We recommend using a color that blends well with the surrounding area. White, black or grey usually work very well.
For rear projection, there is specific screen material designed for this purpose. We have also found that a frosted shower curtain liner is an excellent surface. It is a smooth, consistent, translucent distribution, reasonable size, and inexpensive. Sheet plastic can work, but it often has variations in the translucency.