When to buy a nursing bra? Is a tank a substitute?

songs4drowning

I've been wanting to go pick up a nursing bra but I'm not sure if it would be a waste at this point. During pregnancy, I did not have the fantastic growing boobs - mine have shrunk from a small C cup to a solid B. I'm worried that if I go and get a nursing bra that fits right now when my milk actually comes in (I don't believe I've even been leaking colostrum yet) that my boobs will have a crazy growth spurt and I'd have to go out and buy yet another bra.

I have seriously sensitive nipples so not wearing a bra isn't much of an option for me without dealing with a lot of discomfort so I'd like to have one from the start, but I've been thinking about getting a nursing tank if that has enough support to keep things from rubbing the wrong way? I just don't even know and could use some advice!