You can find them at Sears, JC penny's, Wal-mart, Kmart and in some cases even Dollar stores. It depends on the look and how much you are willing to spend as to where I would shop. Also try on line at Overstock and Amazon.
Believe it or not Wal-Mart carries a lot of curtains, solid colors and designs. Another place that you can find curtains is Ross, and Kohls. Ross is a little bit of a toss up, they might not have them in stock all the time.