You can usually find sales online at various stores. Depending on what you are looking for, it might take a little research. You will have to decide if you want wool, superwash wool etc...Most come in worsted and bulky weight. I think Plymouth Encore is a decent yarn with a lot of colors to choose.
), has a good selection of various yarns.
is also a good choice, www.morethanyarn.com
is also decent.