Are you asking about back-stops? There are any number of possibilities available to you. Basically any backstop that is appropriate for archery will also be good for a crossbow. Flattened cardboard boxes stacked up (put some plastic on top to stop the rain), a couple bails of hay, or straw. I've even used a big plastic trash bag filled with a giant blob of canned spray-foam insulation. Stopped arrows and bolts.