What makes bra sizing difficult is that in reality many more more factors than just cup size and underbust circumference come to play. The shape and length of the inframammary crease, torso shape, breast angle and height are just a few. Fashioning a quality brassier requires quite a bit of engineering skills, which is why good undergarment designers are few and far between.
This also means that most manufacturers just cannot afford to cater to every body shape. I once went to Victoria's Secret when passing through the U.S., as I'd heard many people praise them. However, to tell the truth I was surprised at how limited the offering seemed. Although they had flashy designs, it seemed to me that the company was just a high-priced mass market retailer—and to be quite honest I thought people must be buying from them more because they have good marketing staff than because of product quality.
In contrast, while few, there are some companies that circumvent the sizing problem by stocking just the parts and assembling them after fitting the customer. The largest where I live is Wacoal. Their Dublevé line offers 3,030 size variations, and the cost—from $70 to around $150—isn't that bad either, if you consider the time saved and disappointments avoided. After all, what you get is guaranteed to fit perfectly every time.
This of course means that to make the initial purchase you must first have a fitting done and then wait a few days for the item to arrive. They do keep the sizing data, though, so you can then make further purchases online. (Of course this only applies as long as your measurements don't change...)
