I have maternity clothing from quite a few stores. It depends on how much you want to spend. Motherhood Maternity and stores like it have excellent selections of all things maternity wear, but they tend to be expensive (although I have had success finding awesome items for $2 or less on the clearance racks). Target has a nice little section of maternity clothing for less. JC Penney has a small section as well, and their prices are in the medium range. I went there for my maternity work clothing.
Maternity clothing can also be found online on Amazon or eBay.
If the baby bump is not that big, you can get away with regular clothing. Many shirts these days are fitted right below the bust line and flow at the waist, making them great for pregnancy and post-pregnancy wear. Others are not fitted at all and can be worn with a belt to accent the smallest part of the body above the belly.