Check the evilBay too. I've bought good heavy proofed canvas from there as well as Profabrics.
Watch out with the fabric weights on profabrics. They have some waxed canvasses, but I never got an answer from them as to whether the weighing was pre/post waxing. They do have a free swatch service although when I ordered some they never came - I didn't bother chasing though as I found what I want on the bay with a great company that provided swatches of everything I was interested in.
Profabrics are a good company though and usually spot on.