I have bought designer denim from Kid to Kid in Plano, they have a ton of 7FAM and Joes and TR, every now and then I have seen nicer maternity designers, but most of their other clothing is the normal mall brands.
Clotheshorse Anonymous on Preston/Forest has some (Isabella Oliver, Paige denim, Citizens). I also found quite a good lot on Craigslist for designer maternity clothes.
Re: Maternity Clothing Consignment
Our Wedding Film. | For sale, yo.