The duo- fast stapler from United is the best! I think the staples have a 1/2" crown and 3/8" or 1/2" length. You can adjust depth too. One of my clients tried it once and I had to get him one he liked it so much. Especially good for large canvasses where a pneumatic tool is cumbersome.
Arrow T-50 Stainless Steel staples. They also have a nice light air operated gun, sells for $35.00. The staples lay smooth with out tearing through the canvas, the visibility of the staples location is great. A nice tool for the price.