Ebay, Goodwill, and consignment shops. Search online for maternity consignment shops in your area.
I use to order most of my stuff online through Gap and Old Navy, they always have sales.
I first got mine at target or motherhood maternity which do add up, now that I am towards the end of pregnancy, I figured out ebay has a lot of maternity clothes and are decently priced, I got a shirt for 10 bucks plus shipping but they have some items that have free shipping. Good luck.