I suggest you look on ebay. I was able to pick up a "lot" (5 shirts, 1 pants, 1 skirt) for only $12 plus shipping. Shipping was like $6. Not a bad buy for under $20.
I had a terrible time finding decent maternity clothes and like you, money was tight. K-mart (Barry & North Oak) had the best selection of reasonably priced new maternity jeans. Major Thrift (Claycomo) also had a decent selection compared to all the other Northland Thrift stores I tried. However, Burlington Coat Factory (Antioch Shopping Center) has a great selection of new clothes and you'll be surprised at how cheap they are-especially compared to other retailers in the area. It was the only store I found in this area that had nursing bras and they were under $10. Check out the clearance racks at Burlington while you're there. I swear, the prices will blow you away!
Good luck!