Don't just buy a bigger fashion bra. Seems like that would be an easy solution -- just size up a cup and add an inch or two of girth. But honestly, that doesn't solve your problem. The fabric may fit around your enlarged breasts, but the design and construction of a fashion bra doesn't provide the support necessary for a pregnant or nursing woman.
Buy more than one. Experts say it's important to have at least two or three and launder them often for optimum health benefits.
Be prepared for two trips. If you're buying your first maternity bra in your first or even second trimester, keep in mind that you may have to make an additional trip in the final weeks. Resist the temptation to buy your first bras with "room to grow." They won't fit properly at the onset. Better to just resign yourself to the idea that you may outgrow your first maternity bra.
Consider nursing bras. They offer the same support as a maternity bra, and they have openings to feed the baby through. You may save yourself some money by going this route, even if feeding your baby seems a long way off.