I use HP's Canvas Textured Paper as an inexpensive alternative paper that vaguely resembles canvas. I'd like to find something with a heavier texture, but it is the closest thing to canvas that I have used personally.
As to Dan's question about what differentiates paper and canvas when they are from the same fiber source, I'd say that it is a matter of whether it is woven or made from pressed fiber. Paper is made like felt, and the fibers are randomly pressed together. Canvas and other cloth has a warp and woof (or weft, if you prefer). Cotton is a 20th century development for artist's canvas, and many art purists still prefer flax as the fiber source (linen) and consider cotton to be a cheap alternative. Although I do like the look of linen canvas, poly/cotton fiber makes the most stable and consistent canvas. I don't think many people will argue with you if you call any heavy, somewhat course flat woven fabric with no bias that is suitable for stretching "canvas". All inkjet specific canvas that I am aware of has an acrylic gesso layer as well as an inkjet receptive coating over the gesso.
