Hesco (http://www.hescoinc.com/) has them pretty cheap, though I have no idea on the quality. (Model numbers to look for are EL-8000 and EL-9900)
I bought mine from GoVacuum.com, on the other hand, and it's been pretty okay. The swivel isn't as free as I'd like, and the vent is too easy to open, but it's in much better shape than the woven one I had that came with my Diamond J.