Beef netting spiderweb is available from Trenton Mills. It comes in rolls of fabric which you can cut to your desired length and make the holes as large, small, and numerous as you'd like. Unlike the stringy stuff available in stores, this webbing is reusuable from year to year and won't tangle up wildlife.
It tends to sell out in October, so buy it a few months ahead to be sure you'll get it in time.
Webbing isn't all it's good for; with some paint, pumpkin fangs, insulation foam, and an assortment of christmas ornament blanks painted to look like eyes, you can make a fleshy abomination too!
These mushrooms are made of pool noodles with a PVC core, strofoam for the caps, and wrapped in saran wrap which was heated to make it shrink and seal tight.