The key to finding a bra that fits is to be measured by someone who is certified to measure a woman for a bra. Once you have a true, accurate measurement, you'll be able to shop within a much smaller range of sizes.
I was measured at a 44DD. Depending on the style of the bra (for the most part, only the cut really makes a difference in a well-made, quality bra), I can wear anything from a 44D to a 46DD. (For example, in a demi or a push up I wear a D cup, either 44 or 46 depending on how much weight I've gained or lost, and in a full coverage bra, I almost always wear a DD)
I will tell you... If you wear plus sizes, you will not be able to find a bra that fits, even if it says its your size, at Walmart, Target, or any discount store. I've bought countless 44 and 46 Ds and DDs at Walmart, and not a damn one fit right. I'm wasting the money I thought I was saving by shopping at Walmart... Lane Bryant has absolutely the best bras ever, especially if you like bras that worship the boobies... Sexy, cleavage enhancing bras are the best at Lane Bryant. Avenue has some really nice bras, but for the most part you won't get any really cleavage happy bras there.
You can get measured, and accurately, at Lane Bryant. That's where I got measured, and now that I have been measured, I'm much, much happier with how my bras fit.