To be honest, you can use just about anything. I've seen them made of wood, cardboard, paper, metal, glass, plastic, assorted fabrics, etc. If $$$ is an issue, start looking around the home for junk that you can recycle. Old rain jacket with a hole in it? There's a puppet or two. A bunch of empty tin cans? A robot puppet if you don't own tin snips, anything at all if you do. See what I mean?