An excellent collection of crowdsourced measurements and information on different makes/models. A great first stop to check out fits, models, makes, and get advice. Also contains listings of bras for sale/exchange/giveaway.
Places to buy nonstandard bra sizes
ASOS – Free shipping both ways!
Bare Necessities – Selection is pretty limited, but they ship fast.
Belle Lingerie – ships from the UK, and currency in pounds. You’ll need conversion if ordering from elsewhere.
Brastop.com – The makers of Curvy Kate have lots of amazing deals, but shipping is expensive to the US (unless you spend 75 pounds, a big chunk of change) and can take up to a month in my experience.
Bravissimo – Has customer reviews. Can be expensive, but has own lines which are apparently quite good.
Breakout Bras – Ships from US. Good commentary on every bra!
eBay – recommended only if you already know the size/model to order, as return policies may differ or be nonexistent.
Ewa Michalak – A Polish company, you will need to switch to English at the top. Shipping to US takes a few weeks.
FigLeaves – A UK company, but fast shipping and reasonable charges. They are good with returns too, and sometimes you can get things on sale.
HerRoom – I don’t like their nonreturnable sale policy, but there are a lot of excellent bra reviews and data for informational purposes.
Large Cup Lingerie – free international shipping and sales, too!