Are you doing a standard wrap (1"), gallery wrap (2") or museum wrap (3")? most art supply stores will only carry 1" stretchers at a reasonable price. If you are going for a 2" stretcher, I make my own just by taking my table saw and cutting the drop edge with that, and cutting my 45's on a mitre saw, with standard wood glue and brad construction. here is a little image I made for a buddy who was starting out with making stretchers himself (museum wrap 9'x15' canvases .. yeesh.) if it's large, adding support corners and support bars help reduce warping over time. Basically the idea for a stretcher is to provide a drum stretched surface, so as small as you can make the wood touching the canvas without being weak and breakable is the ideal. one other small thing I do is cut the corner so when you wrap the canvas on a digital print you aren't doing a double fold on the print, simple V cut does wonders for that.