The answer to this question is going to depend on what size you actually wear. If you are a plus size, which is to say a band size of 38 or higher with a cup of D or above, then Lane Bryant might be a good place to look.
If, however, you are like me, a "regular" band size (under 38) but a larger cup, then I would look on figleaves.com or breakoutbras.com. My favorite brands are Freya, Fantasie and Panache, they are all made in the UK and have slightly different sizing, but they make bras that are supportive, cute, sexy and in cups up to K (or even higher). Breakout bras also has a great section on bra sizing that helped me a lot. I was able to figure out my correct size at home and the difference was amazing.