The ideal sports bra needs to have compression to keep movement to a minimum. If you are making milk, it also must be comfortable- i.e. no squishing those precious milk ducts with an underwire or likewise. The freedom of movement afforded once you find the perfect sports bra is unparalleled. Not only can you keep making milk (if you so desire), but you can MOVE. You can run, or dance, or do aerobics, or yoga, or whatever-- without those breasts getting in the way.
This is the Champion Smoothie. I include it because the straps are adjustable, and they cross in the back or not, depending on your preference. It is also sized like a bra, and comes up high in the front, so your cleavage stays modest.
compression bra from Target. At a time when I was multiple sizes on any given day, having a bra sized S-M-L-XL was more useful than one with a cup size. They are also inexpensive, which is useful when you only need a size for a few months.
These are a few ideas-- so you can get out there and MOVE!! Go buy a couple, and see what works for you.