Joann fabrics, Walmart, Kmart... They all sell it. It should be in the hobby/linen isles. It's just poly fill made from nylon material. Looks like cotton. The more you put in, the more you "trick" your subs to think it's a bigger box. Usually half way to 3/4 works. I have my two subs stuffed about 1/2 way, finding 3/4 just took too much "oomph" out of the bass. It eliminates reflections inside the box (which is a good thing).