Where to buy usually depends on your band size. I'm more of an unusual size. 34 DD/DDD. Most of the time I have to settle for 36 band if I need a bra in the moment.. Why the popular bra makers don't make more of the smaller band sizes with larger cups, I don't know.
Because of my band size, sports bras are hard to shop for. Most of the time, to get them big enough for the cups, my boobs fall out the bottom. It's a mess. So, I don't have much in the way of recommendations on that. I figure that I probably should stick to low impact exercises. ;)
Wacoal makes a good bra for the bigger girls. Not cheap, but you can get them at Macy's, Belk, or other department store.
Figleaves.com has some awesome sexy bras in larger cups. The downsize is that you can't try them on, but they have a decent return policy. I have also bought bathing suits from this company and was very happy.
Hope this helps.