Sara, you'll find, as you go, that there are a few brands that you prefer. Once you know that, then you can go and confidently look for those brands, whether they're new and full-price at a LQS, or whether they're sale or past-season prints that you can find at the discount places. If you're shopping online, then it's really very important to shop from a place that does tell you who the manufacturer of the fabrics is!
One thing I've noticed about the fabrics at JoAnn's: the designs lately are very nice... but you have to unroll the fabric and carefully inspect it, for 3/4 yard or so, to make sure that the printing itself is top-notch. I've found a lot of times there are little unprinted spots that repeat every so often throughout the whole bolt. If you're doing patchwork, you can probably work around that sort of thing, but if it's for backing or even borders, then you'll have a problem.