In stretching the canvas, I put in 12 temporary staples (without using the stretcher pliers): one at the center of each side, then two staples at each corner, just to have a bit of tension throughout.
The canvas comes from Dick Blick in Berkeley. It's medium-weight, 100% cotton duck. I don't need the smoothness of linen, so cotton works just fine. Incidentally, the folks at Dick Blick used to let me use my 40%-off coupon for any number of yards. Now they just give me 40% off the first yard. It was nice while it lasted.