I have tons of sources, but it would depend on what type of yarn you wanted and what you were using it for. If you just want plain old wool yarn, you can get natural, undyed Fisherman's Wool, which is 100$ wool, at Joanne's. It's not the softest wool in the world, but lots of people use it and like it well enough. I just used a skein of it for my spaghetti for the wool foods swap. If you want merino wool, I would suggest blackberry ridge. It's a lot more expensive than Fisherman's wool, but it is so soft and wonderful to work with.
http://www.blackberry-ridge.com/ Another wool I adore is Blue Faced Leicaster and I get my BFL from Wooly Wonders by Nada when she has it instock
www.hyenacart.com/woolywondersbynada . It's also a lot more expensive but totally worth it, depending on what you are using it for. I also searched on ebay and this is the list I came up with:
http://search.ebay.com/search/search...rclo=&saprchi=