A dog bed that we found at Big Lots served as the body. We've sprayed on some black paint to mottle an otherwise red bag. Pipe insulation - or spider legs, as we like to call it. The key ingredient for poseable legs would be 9 gauge wire. Needing a viable means to secure the legs, we procured a replacement grill for a barbeque.
Cable ties hold the leg wires in place.
Here we have a garden hose pot, in the process of becoming the lower portion of a spider head. The closest I could get to store-bought spider fangs was a pair of glow in the dark machetes. They were summarily butchered. Black spray paint is our friend. Jeff there belongs to our neighbor.