I am also fairly flat chested (34A) and find the Target bras fine for running. My DD wears a much larger size and sometimes finds good sports bras at TJMaxx. They carry good athletic wear, but obviously it is hit or miss with sizes. But the Adidas bras I bought her at the department store when she was in HS, for $34 can be found at TJM for about $15. Check them out!