Check out Lands End overstocks - there are sometimes a lot of maternity clothes on there! And you can't beat the quality. Sears, JCPenney and Kohl's are hit or miss but at least have sales and a decent return policy.
I don't like Motherhood as their quality is really subpar and their return policy is even worse (at least, it was 3 years ago - something like 7 days, exchange only, and item must still have tags on).
Also check children's resale shops and other thrift stores. I've found some fantastic maternity deals at stores like those in the past! eBay is always an option, too, but I like to see and feel "used" clothes before I buy them.