Fabric.com!

I'd specifically recommend their harem cheese cloth, which is a full 56" wide - not like the narrow, packaged kitchen stuff.

 

http://yourfabricsource.com/cheesecloth?zenid=96suhls2d5as7uoaiv3a7k3cf5

I buy the unbleached so I can dye it more easily or undyed for costumes. The box lasts a long time!

 

The Royal Bargains dollar store chain has halloween oriented stuff called freaky fabric for a buck . Comes in grey, green and black colors. I seen the same stuff on eBay for over $5