Target is amazing--the only store I've ever found where I can just walk in and buy bras in my weird size, 40A. They fit exactly right too. 40 is the girth of my huge rib cage, A is for my teeny titties. I'm sooo glad Target is there. It's actually easier to find a variety of pretty bras in my size in the store than on the internet, the reverse of what you'd expect.
For me, wearing a bra has turned out to be perhaps the single biggest difference between passing and clocking. It defines the shape of the bosom according to a standardized outline. When a person presents female, the public's eyes go directly to the bosom. If you're a big gal and flat as a board, that screams "man in a dress." I noticed I started passing much more easily as soon as I had a bosom to present.
Bra: Don't leave home without it.
Another factor you should consider in growth is getting enough sleep, 8 hours every night. HGH production depends on sleep, so if you aren't sleeping enough, your breast size may suffer.