I'm not an expert on this by any means, given that I've never harvested, but I've read about it a bit, so I'll give an answer to get the discussion going on this interesting question. The "yeast" chapter of How to Brew touches on this subject.
You can harvest the yeast from any beer that's bottle conditioned. For instance, I think most Belgian Trappist ales have harvestable yeast. The last Chimay I had sure had a lot of sediment in it. However, I don't have a complete list and I'm not sure which, if any, filter out the primary yeast and then add a different strain for conditioning.
Another option for harvesting commercial yeast is to visit a local microbrew. Become friendly with the brewer there. He may be willing to give you some of his yeast, as they tend to produce more than they can use. I can't take credit for this suggestion. It, too, comes from How to Brew.
Best of luck!