More ways to get cheap yarn!
elann.com and WEBS, there is another resource for cheap yarn: discount stores. These stores only sell yarn as an afterthought, and the supplies are usually inconsistent at best.
Your best strategy here is that if you see something you want, you should buy it. You can't count on it being at another store, or even being at this store tomorrow. And you may literally never see it at this store again.
However, be strong against the tendency to let this fact throw you into a yarn-buying frenzy! Decide ahead of time what you will and won't buy, and the absolute maximum you're willing to spend. Otherwise you can find yourself in big trouble, fast!
Outlet Grocery Stores
There are several grocery chains around the country which buy off-brand and near-expiration food. We used to call these "dented can stores" when I was a kid and botulism was funny. (???) I frequent one of these stores, and I have seen yarn for sale there on occasion. It tends to be the ultra-cheap novelty stuff.
These days you can find yarn at stores like K-Mart, Fred Meyer, and even some Rite Aid stores. The yarn isn't usually discount priced on a regular basis, but it will often go on sale along with other store items (partly because no one knows it's there, so their through-sale rate is probably pretty lousy).
True to its name, every Tuesday Morning this chain of discount stores sets more stuff out on the shelves. What kind of stuff? All of it. All the stuff. From dining room sets to plaster dashboard Jesus statues, you can find it here. Yarn included!
A recent Tuesday Morning announcement that they would be receiving a load of sock yarn caused something of a frenzy on the Ravelry boards. (For the record, only about a third of the stores visited by knitters proved to have sock yarn. Shopper beware.)
This chain is similar to Tuesday Morning, although in my experience they tend to focus more on "giant bins of random objects." Big Lots stores reliably have SOME yarn, although it may not be more than a few skeins, and it may not be a brand you've ever heard of. Big Lots sometimes sells name brand yarns like Lion Brand, but they will tear off all the ball bands, so it's kind of a mystery purchase. Fun?