In my print shop, we order stretcher bar material from this company: http://www.linenliners.com/products/stretchers/
We get full bars and cut them down to size. We offer completely custom sizing, so it's easier to just order the long sticks and miter them to the custom size and join them together. The wood is high quality, very smooth, no knots or rough areas, no warping - they finger join the wood together, in 1 foot sections, to minimize the potential for the wood to warp over time.
So, if you want to order your own material, cut it down and build your own stretcher bar frames, that's the place to go.
If you want a place to stretch your canvas for you, I'd start with a local google search. Just google "Canvas Stretching XXX" where XXX is your city and state. You should come up with a few options. It's not ideal, since they're expensive, but even Michael's and some other hobby shops will do custom framing / canvas stretching as well.
Try to avoid sending the canvas out to be stretched. Not only will you have to pay the stretching fee, you'll also have to pay the shipping to the stretcher's location, plus pay the shipping to have the larger, stretched pieces shipped back to you. Shipping can get very expensive, depending on the size of your canvas.
If you have any other questions regarding canvas stretching, please don't hesitate to ask. My contact information is listed below.