I ordered most of mine from Old Navy, and I returned anything that didn't fit to the store so I wouldn't have to pay to ship things back.
Walmart.com has some cute stuff. They are way cheaper in regards to maternity sleepwear and nursing bras. You can even have your order sent to your local store to avoid paying any shipping. You can even specify one other person besides yourself to pick up your pre-paid order. The savings on shipping really helps.
Target and JC Penney have some cute maternity clothes. I also went to stores like Ross and Marshalls, but toward the end of the pregnancy, my patience was too thin to sort through the racks.
Where I live, we also have a Motherhood outlet store. They have great customer service and good prices.