Just make sure that your taking your underbust=band size as a starting part so you get enough support. You might be interested in these blogs:
http://broodsbigbrasblog.com/
http://branightmares.blogspot.com/
http://fullerfigurefullerbust.wordpress.com/
all of them show the fit of lingerie on plus-sized figures and give a lot of tips on buying lingerie. The author of broodsbigbrasblog actually owns an online store that sells lingerie (of all sizes not just plus-sized) and she models the lingerie so you can see it on a real model, just just someone perfect.
You also might be interested in the Curvy Kate Star in a Bra competition. It takes women of all ages/sizes and the winner gets a one year modelling contract. Right now they're voting on the top 30 and it includes girls from a UK size 8 to UK size 16 (so about a US 4-14 not the widest range but they didn't have a lot of women entering at the higher end). You can see the models all carry their weight differently and they aren't all the standard model figures either (some are short, some are over 30 etc).

Don't get my wrong, I'm pretty happy with my body but nobody would pick out my body to be a model.