I can highly recommend Stoo's canvases. I picked one up from his a week or so ago and the quality is second to none. For a canvas of the sizes he's quoted above then his prices are very reasonable for the high quality you get.
Your canvas from Stoo will come stretched on to a very high quality wooden frame that is guaranteed not to twist or warp. Stoo describes it as ready to hang which it is apart from attaching some hooks or string to the back.