You can buy undecorated grave blankets and decorate them yourself. You can make them from scratch, but there is a bit of work involved.
You will need a piece of rectangular styrofoam to start. If you want a small blanket you can leave it at that. If you want a full size blanket you will have to frame the foam with wood to support longer branches. Start pushing evergreen branches(choose an evergreen with sturdy branches) in at the corners and sides of the foam to set the shape. Continue until the blanket is filled in. After you have your base, you can fill it in with Fancy greens as accents.
To decorate it start with a bow in the center and fill in around it. I prefer natural decorations such as holly, pinecones, and dried status. To attach the decorations run a wire around them and wind the other end around a small pointed stick. This can easily be pushed into the foam. Many craft store have decorations already on picks that you can use.