Hi J.!
I loved Motherhood Maternity, Target (Maternity section) and Old Navy for plus size Maternity clothing. They both have good sales and crazy clearance prices (especially MM). Some stores are better than others so you may have to visit a couple to find a really great selection. (I like the Meyerland Plaza location although it is quite a drive.) I kept it simple and bought mostly classic pieces so I could mix and match easily. I was able to wear them through both my pregnancies.
(Also, I found maternity/nursing bras very uncomfortable. I began wearing sports bras, during pregnancy and nursing, which were so much more comfortable and gave good support.)
Good luck and CONGRATS!!!