Probably not very helpful, but shops where they measure you - i.e. with a tape - to work out your bust size aren't much help.
If you go to a really good fitter, she will look at you and bring you some bras to try, and work from there. Lovely to report, we're all different, and a skilled fitter not only assesses your own size but will know how the brands she sells come up in size, sometimes going up or down a cup size or band size to achieve the elusive 'perfect fit'.
I get fitted regularly, as being a man with boobs I don't automatically fit into a given size, depending on style, make and so on, so being fitted is a real help. In so far as I'm a size, I most often take a 38C, but I've got bras in 38D, 38B, 36DD, 36D that fit me just as well.
I feel for you, as you're not in the 'normal range' that dress-sized underwear is designed to fit - I have the same problem at the other end of the size scale! If it's just the extra volume of the cups, you can always try 'chicken fillet' enhancers - I use them because cups that fit my 'manly' chest tend to have a bit of room left in them.
Otherwise, if you can find bra-sized baby-doll and chemises, you may do better, with the proviso that you may need to swap once or twice to get sizing absolutely right. I got Mrs. P a lovely bra-sized baby doll from Bassoni (Oz brand) but I can't find another one like it, so the search may last some time!
If you're a size 6-8, you may be about a 30C, which would be more like a 34A in cup volume. Do you find it hard to get bras small enough for you?