It's best to wait and shop for nursing bras until the third trimester, preferably in the final month before labor & delivery. At that time, you should plan to purchase 2-3 comfortable bras in order to be prepared for the days and weeks after labor.
Keep the following tips in mind as you shop:
1. Don't buy a bra with underwire. Underwire is uncomfortable - it pokes and prods in all the wrong places. If you are concerned about finding a bra that has support, you can rest your mind at ease. There are plenty of seamless, stretchy bras out there that also have adequate support.
2. Go for comfort. Yes, feel free to let the well-meaning sales associate size you, but don't feel obligated to buy the size they recommend. Buy the bra that feels right to you - regardless of size.
I've attached a photo of my favorite maternity bra to this blog. It's from Kohl's: the Oh Baby! by Motherhood Seamless Nursing Bra. (If only they sold it in black, I'd be oh-so-happy.)