Good question! I had the same problem. While there are bra back extension bands on the market, I have not come across a wonderful bra with a cup size that grows with you. For that reason I had to purchase a number of bras during my pregnancy. I found out the hard way that you get what you pay for — budget bras I bought were quickly replaced with quality, professionally fitted maternity numbers with ample support and comfort.
You may not be able to avoid the expense of buying bras, unless you have a friend like me who passes on her Amazonian intimates to pregnant friends once she’s done with them. But you can at least find some solace in the notion that what goes up must come down, so you’ll be able to wear the sizes you outgrew when you begin to lose the baby weight.
One thing you should consider if you need a new bra and are planning to breastfeed is a nursing bra instead of a regular maternity bra. A quality nursing bra will support you until you have your baby and it will become even more useful after the fact.
Whatever you do, make sure you buy a great support bra fitted by a pro. It may be expensive and about as sexy as grandma underwear, but as you can probably already attest, nothing trumps comfort when it comes to the ever-expanding bust line of a pregnant woman.