Often, wearing these under a business or business casual jacket is just as good as a button-down. I find three button (as opposed to 2 or 1 button jackets) work really well for larger boobs, and I've found suitable jackets like this at Bravissimo, Victoria's Secret Online, and even Target (in the Converse One Star or Merona brands).
When I really need a buttoned shirt, I tend to go for bravissimo or a regular button shirt that can be altered. Actually, even the bravissimo shirts I usually alter with my sewing machine because the length is usually wrong for me. Buying a large shirt that fits the bust and altering the rest is another option, but not always feasible. In addition to being a lot of work, altering larger shirts down can be dicey because it's very difficult to make wide shoulders narrower and that will ruin the look of the shirt.
So the trick I've been using lately, which works surprisingly well, is to find a button shirt that almost fits in my normal size, but gapes a bit. Then I just sew the gaping parts shut with my machine (with two lines of seams--one on each existing seam line on either side of the buttons). Then I put the shirt on and off over my head--if the fit is close, this technique can work really well, especially since cotton shirts will stretch a bit with wear, and without the issue of gaping, it's usually not a problem if the shirt is slightly (and the key word here is slightly) tight to begin with.
I recently found some shirts that fit me well at Target made by Converse
that have stretchy side panels made of cotton rib, and then some other Converse shirts that almost fit in the boobs and fit me well everywhere else. I got my normal size (medium) and did the sewing shut trick on them. These latter ones are slightly see-through though, so you might want to wear a cami under them if you are not wearing them with a jacket.
This is a pretty simple trick, but I hope it might help--it worked much better than I thought it would. I think the key is using a sewing machine to make the stitches neat, invisible, and strong, and sewing along existing seams. This is something I have thought about for a long time, but never done before because I was afraid it would look odd. But honestly, I decided to give it try finally when I saw the leading lady in the show Warehouse 13 (on the Sci Fi or "SyFy" channel) has this done to all her button-downs, even though her chest is not large. I guess the costumers decided to do it so she could move freely without ever having to worry about buttons pulling open. So I am pretty impressionable, apparently--at least when it comes to science fiction tv programming. (Surreptitiously removes Vulcan ears and hides them in couch cushions...)
One other tip: I also find wearing jackets (business or business casual) over button-downs really helps improve both the look and confidence of most citizens of bigboobtopia. Buttoned shirts, particularly white ones, can be VERY unforgiving, even when they fit relatively well, so a structured jacket really helps pull the look together and make everything look neat and sharp.
Next time I will be reviewing a few seamless bras I got recently, including the new Freya Ava Plunge.