Depends entirely on the printer and the type of canvas. Go to the manufacturer's web site or ask them what they recommend.
There are books you can get in the library; a technique for fabric printing is to stiffen the fabric with size press it absolutely flat, then bond it to waxed freezer paper with an iron. The fabric/paper combo then goes through the printer like card stock. You have to have a printer that has a straight through paper path, and that can handle heavy stock.
Alternatively, you can go to a service bureau or print shop that has an Iris, Roland or other large format printer that can print on a variety of fabric stocks.
Oh - did I mention the part about checking whether your inks are waterproof?
There are definately some companies that do print your pictures onto canvas, but I can't think of their names. I'm sure I saw them advertised on a website like Jessops as a 'gift idea' but I don't know cost/size/etc. Just to let you know ...