Back in high school, I played sports and it was suggested to me one time to wear two bras at once. I tried it then but it doesn't work for me. I have extremely sensitive skin (eczema), and my skin was breaking down from the pressure and friction of the two bras. It could work well for others but not for me.
I'm not sure if you could find them online... But I have a sports bra called the "Ta-Ta Tamer" (haha... I know!!) :P
I found it in a yoga store in the Charlotte NC mall. I'm a 34 DD, and it squishes them in. Lol At first it was uncomfortable, but eventually I realized that having them lifted up actually helped my breathing.