As someone who wears at 30FF(UK)/30H(US), I feel your pain . I mostly get my bras from online as Nordstrom and specialty bra shops are the only places in the US to buy in my size and the prices are outrageous. Most stores (at least in the US) only sell a limited size collection and will fit you into a poorly fitting bra to make a sale. Victorias Secret tried to fit me in a 36D which is three band sizes too big and one cup size too small (remember every time you go down a band size, you go up a cup a size). Here are some of the places I shop:

Brastop- its a UK site but you can get bras for $25-30 bucks per bra (plus shipping) and they often have shipping deals for the US. Great return policy as well
Figleaves- another uk company but also has a US site so it ships quicker. Tends to be a little more expensive than Brastop.
Amazon- a bit of a mixed bag but I've found bras that usually go for $70+ in the US for around $30-$40 and Amazon often has free shipping.
Bare Necessities- also sells DD+ but partners with Amazon. Last time I ordered Bare Necessities bras, I ordered thru Amazon. You will also have to shop the sales as their regular prices are $50-80 per bra but their sales are usually over half off. Also they are currently offering free shipping.
Nordstroms Rack -can also have some deals but tends to be limited selection

Since you went to Nordstroms you are probably in the right size (as they are about the only place that even comes close to properly fitting women) but I would remeasure on your own just to make sure. Make sure you DO NOT ADD INCHES TO YOUR UNDERBUST. The 4+ inches rule is an outdated model of measurement and was created before the use of elastic in bras. Most women cram themselves into ill fitting bras because bra companies never changed their sizing to accommodate the elastic in bras. Issues like not being able to bottom up tops, back pain, shoulder pain, rashes in the bust area, quadboob, arimpit fat, back fat etc. are generally actually due to the bra .

You can use Sophisticated Pairs calculator at sophisticatedpair dot com slash bracalculator dot htm to find out your size. Most places that sell above DD use UK sizing so I would definitely find out your UK size at least. Also remember that you made not be the same size in all brands as bra companies also unfortunately participate in vanity sizing. For example, the Freya deco tends to run large in the band and cup and Curvy Kate in general runs small in the cup (especially for the showgirl collection). So don't be surprised if you're one size in one brand and a different one in another.

Lastly here are some signs of a properly fitting bra:

the back band lays flat/doesn't ride up
the center gore lays flat on your chest
all the breast tissue is in the cup
no quadboob
no discernible armpit fat or back fat (this is actually migrated breast tissue from wearing poorly fitted bras!)
the straps are not digging in
you can only pull out the band 2 inches at the most
there is no wrinkling in the cup
your bra feels supportive without being too tight
when you take off your bra marks that go away are fine, bruising/pain are not

Have a great day!

fixedatzero