for a bra, go to nordstrom's. get measured and then invest. you may be surprised about what they have to say about your size. don't hesitate to ask any questions at all! especially if you think they're crazy for saying you're a size...whatever it measures out to be.
about the milk supply. you mentioned feeling like you had a lot of milk and not so much now but still enough for your baby. that's all you need--enough. when exclusively breastfeeding our bodies are pretty good about meeting our baby's needs.
yes, secure fitting bras can decrease milk production, but not always. wear the secure bra for your work outs and then switch out when you're done. as far as the running itself decreasing your milk supply, it depends on how much you do.
as long as you continue to feed your baby and are able to provide all that she needs, you know your milk supply is good. if you're burning so many calories, your body may get confused and slow down on milk production.
keep in mind, our bodies are designed for survival. so you may or may not keep on a few pounds as long as you are nursing. this is your body anticipating any possibility that you might be left on a stranded island, or in an earthquake , or flood and can't get to enough food. so what your body does is keep on a few extra pounds so just in case, you can keep you and your baby alive for a while.
in my case, keeping the weight off was the easiest for me when i was nursing and am hoping to use that to my advantage in the coming months.