However, more reasonably priced for a project like this, and easier to find in a local home supply or hardware store is what is specifically called “plaster of Paris.”

As you are pouring the plaster, halfway through, it is always best to place pieces of sticks or short thin but strong strips of metal into the drying plaster, then pour more plaster over them. This strengthens the finished plaster copy.

This is only a shortcut answer, and the fine and detailed technique of casting tracks, especially of Bigfoot and other cryptids, is a study unto itself.

Actually, casting your own tracks is good practice before you try to take a cast of something important and unique in the field. Even if you don’t send me your casts, I highly recommend you experiment with trying to make casts of your own footprints or your relatives. Ignore their laughter (as I did when I was 12), and master your technique in your backyard before hitting the woods!