Dick Blick and Jerry's Artarama both have assembled floater frames up to medium sizes. And I'm sure there are others. If you use a lot of these track down wholesale sources as well.http://www.jerrysartarama.com/discount-art-supplies/framing-and-matting/ready-made-wood-frames/illusions-floater-frames.htmhttp://www.dickblick.com/search/?q=floater+frames&x=-620&y=-127&sp_cs=UTF-8
If you want to chop and join them yourself, Omega will usually be your cheapest option.
Am thinking some of those prefabs may be polystyrene, not sure. No big deal, the best of the new poly's look pretty good, while the worst don't.
The one issue with prefabs is that the thin air space between the face of the frame and the canvas needs to be pretty consistent all around for a decent look. The final dimensions of stretched canvases has a little wiggle-waggle factor, so it can be tricky to make that inky black outline look even. The most reliable solution is to cut either the frames or the stretcher bars yourself, or both. Unfortunately, cutting your own stretcher bars means they will not be easily adjustable to take up canvas slack, although you can theoretically rely on tricks like spritzing the back of the canvas to take up late-developing sags. Cutting the bars rather than the frame requires less cosmetic finesse.