Nordstroms has a large selection of special occasion dresses that can be purchased off the rack. If you want a true "Mother of the Bride" dress you will have to go to a bridal boutique. They generally have them where they have the bridesmaids dresses.
Castle Couture has a great selection - but can be on the pricier side. My mom ended up getting her's from Belle Bridal in Middletown. It's a tiny store but they have a ton of options, and the owner, Kim, is super helpful. They're really affordable there. I'd also recommend True Love bridal over in Matawan - they carry quite a few different MOB styles. My mom just ordered her dress last month, so we're happy to have it checked off. Have fun!
My mother and mother-in-law both got their dresses from The Wedding Store in Andover, NJ. I also got my dress there and the bridesmaid dresses. Great store and awesome people. Plus, since I got my dress there they gave my bridesmaids 20% off of their dresses and my mother and MIL 15% off theirs
Re: Mother of the Bride dress stores