You may be that busty, but I doubt it! I wish I was.
My information from Mumsnet - and they should know about bra fitting - is that you should wear a soft cup padded bra and measure under the bra band, then over your maximum bust, thus taking nipple protrusion into account.
Then each inch difference between band diameter and cup diameter should represent a bra cup size. Thus 36/38 represents a B cup, 36/40 is a C etc.
If you can't face up to an individual bra fitting, then try buying cheap used bras on eBay. If they don't fit, then donate them to the Sally Army.