Yay on your new boobies!
I went out and got a maternity bra when I was about 16 weeks pregnant. It felt much nicer to wear a bra that actually fit instead of trying to squeeze my big boobies into my other bras. I just went to Myer and Big W and bought some Bonds maternity bras for about $25 each. Theyre ugly and boring looking, but the do the job. I've also got a Hot Milk and an Elle MAcpherson maternity bra too, which are much prettier but more expensive.

I found my boobies didnt grow much bigger at all after about 18 weeks, until milk came in.