You can always buy canvas on your local art store, galleries and online. Most of the canvas they sell are pre-primed, meaning it's primed with gesso. You can distinguish it by touching the surface and you feel the smoothness. You can choose any size but mostly on my tutorial I choose smaller like 12x16 inches and 10x12 inches. You can find some on my AMAZON page at very low price.
You can also create your own canvas by following my tutorial found above. You just have to prepare this materials. First you need a stretcher like a 12x16 plywood or you can build a stretcher. Second you need to buy canvas clothe found also on most art store and even on stores that sell garments. Third you need a gun tucker to mount the canvas on plywood. Make sure that the plywood is thick enough. Forth, gesso as your primer but you can actually use household paint like white latex gloss or versatix, classitix and permatix. Its for practice only and don't actually recommend it. It's just becase it's way cheaper than gesso.