Nordstrom for a fitting if you have one. By and large, they tend to have the best trained fitters. Being a fitter isn't easy. And even Nordstrom - some are better than others.
Your fitter should be measuring you twice - once under the bust and again across the nipples. WITHOUT the down jacket. Then, because manufacturers have variations, understand the band and cup sizing one up and one down. If you then at least SHOP for your bras at Nordstrom, the fitter will have a pretty good idea if she's been there a while on how each manufacturer fits. (Though Nordstrom bras are expensive and they only carry expensive bras - so you'll be taking this data to your local department store or Target and seeing what you can do without the fitter - the fitter is going to give you a pretty good idea - or get luck at the Rack - I'm rather impressed with sharbear's find - I can never find my size at The Rack).
Last time I needed a new bra (my previous model had been discountinued), I tried on 30 bras to get one that was comfortable. I bought four bras at $75 a bra, and I haven't bought a bra in four years (I wear mine for a few days before washing them unless I do something like jog in them, then wash gentle cycle and don't dry - but I'm in Minnesota where its rarely hot and humid (and when it is, I'm inside) - when I visited New Orleans I had to wash my bras out in the sink every day).