Getting fitted for a bra is your best bet. I spent nearly 2 hours at Victoria's Secret getting fitted and trying on over a dozen different bra styles. It's not just about size to get a good fitting bra, it's also about how the bra is designed. I've discovered that my favorite is a front-close racerback so the straps don't fall down my shoulders, at all, ever, lol.
And if you're looking to give your bustline a boost for when you do go out en femme, then a good pushup will give you shape and structure, even if you can't fill it up completely.
Go have some fun with it!