It is a pretty sweet day in the blogosphere when I can read two awesome sports bra posts in the same day. At RunnersRambles, I learned about the Zensah bra, and at XLMIC, I learned (or re-learned, actually) about the Moving Comfort Juno bra. (And also about how the first sports bra was two upside-down jock straps stitched together by a frustrated runner. True story. That's it right there. Aren't you glad you live in this decade?)
Several women in both comments sections remarked on the dearth of comfortable, functional sports bras for large-breasted women. (In the "Sports Bra History" article at XLMIC, you can read about how, even once the sports bra gained commercial legitimacy, designers poo-pooed the idea of a version for women in the DD range because they just couldn't believe that there could be that many large-breasted women in sports.) In the grand scheme of things, I guess I fall about in the middle -- the cute little yoga bras are not going to cut it, but I haven't had to suffer the same trials and tribulations of truly large-chested athletes.
The "Sports Bra History" article mentioned a company that I have heard good things about but haven't thought about in a while called Enell. Enell was started by the woman who first commercialized the idea of a sports bra for D / DD / etc. women (which I did not know until reading the article). If that's you and you just haven't found the right fit yet, you might also give them a try. :)