At the beginning of my career I worked for a stock house. It's extremely difficult to make a living at stock. We took 50% of all sales. Frankly, it was 50% of not much. (Although, we were strongly editorial, and less commercial- probably more money on the commercial side) If you can fill your own stock requests, you'll make more per sale. We represented a number of National Geographic photographers who were always on the road, so they didn't have time to fill stock requests. It's been years since I was in this business, but I expect that with all the amount of 'clip art' out there, it has changed dramatically. I'm sure it's become very digitized.
I would start your research with Corbis in Seattle. See what they look for in portfolios. They seem to represent everybody, now!