Many suppliers provide sheets of canvas already cut into standard sizes. But it's a lot cheaper just to get a roll and cut it with a straight edge.
Canvas is pretty easy to cut down because it doesn't have the fierce, one-directional curl you get with paper media. If you want to go deluxe you can get cutting mattes at graphics arts stores that are ruled off in inches. That and a nice Alvin stainless steel straight edge and an Xacto knife and you're in business.
There are subtle differences between the types of images you get with the various brands. But IMHO the main thing is the amount of texture. For instance Hahnemuhle has the finest texture, Fredrix 777 is pretty fine, Breathing Color has a relatively coarse texture and the formerly great Epson Canvas Matte now has the worst texture imaginable.
In general finer textures print sharper. So in cases where you need to make huge blowups of small files, a highly textured canvas helps cover up the digital noise & artifacts. If you are thinking about 8x10'ish prints, go for the finest texture possible to preserve detail. BTW Hahnemuhle costs twice as much as Fredrix. But H. also has much better QC.
http://www.inkjetart.comClick on the "Canvas" drop down menu just down from the top of the page, you'll find listings for both sheets and rolls.