Try going to locally owned shop. They are good because they will measure you properly and help you find whats right. Also because they don't sell just one brand it gives you a wide range of choice. I know also they will always order a size for you.

I don't know how much support you need but have you considered having a corset put into the dress by a seamstress (not lace up of course, a fisheye corset)?
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Buy SPANX, avaiable at most department stores.

Also, there is lingerie plus size store called Cacique, from the same people of Lane Bryant.

Good luck
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Having worked at a Victoria's Secret for two years, I can tell you that the likely hood of you finding a total body slimmer that is going to be supportive enough for your large breasts will be hard - as I am sure you know.

I would suggest that you buy a strapless bra at Macys or Nordstroms in your size that fits you well and you feel that is supportive…and then a slimmer for your stomach and legs - Spanx or such.
The problem with buying a slimmer that includes the bra is that often times the fabric is pulled down because of your body shifting when you walk, sit, etc. So if you have larger breasts, this included with the weight of your boobs means no support at all.
You can go to Macys and Nordstrom and try some on and see if you can find one that will work - they both have good selections and styles of slimmers with built in bras, but I would almost garantee that you will do better with a strapless bra and separate slimmer.
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(JC Penneys specifically caters towards women with larger breasts, so you might find something there too).