I have a quick question: How does one go about purchasing bras at websites?

I've done it before, but only in the exact same styles and sizes I've already recently tried on in a store. Given the variation between manufacturers -- and personally, the fact that I simply can't wear some brands regardless of size (Wacoal) -- I'm really uncertain how to know it will fit and really, really hesitant to spend money on what seems like a gamble.