The online site mentioned above is good, but I would go to a proper lingerie store (not Victoria's Secret!) to have her measured and try on a few options first. Different styles and manufacturers will fit slightly differently. Once she finds some she loves, going online is great. Now that I know which Wacoal size fits, I can go online and order the color I want.
Really, Victoria's Secret doesn't do that well at measuring and fitting bras. I have 5 sisters and we are all different shapes and sizes and VS has mismeasured us all, in different stores in different states. One of those small, higher end lingerie shops will usually measure and fit you into bras that will make an amazing difference. 4 of my sisters, my stepmom and I went to one last December and wow did it make a difference. My 40 DD sister, according to VS the month before, was actually a 36 H and looked so much better, not to mentioned reduced back aches, in the proper size bra. The rest of us all had a big, but not quite so dramatic, change too.