Try a Cheata bra. I’m the last person to worry about chest movement (I practically have none), but I’ve been seeing FB posts show up on my feed. The videos and testimonials seem really impressive.
Definitely see about getting a zipper installed behind the buttons. I broke down and bought a Pikeur with a hidden zipper last year and have not worried about a button since.
I even had a zipper added to my shad until I am ready to replace it.
Good suggestions! I have trouble with shirt buttons gapping at work and sometimes popping open. I sew a tiny snap fastener in there to keep things closed, but that wouldn’t be strong enough for the situation OP describes.
And about buying jackets of shirts that fit across the chest, yes, you can get them large and have the waist tailored. But sometimes that means that they are too large across the shoulders, and that’s almost impossible to alter. And if the shoulders droop, your arms are constricted, no fun.
I hadn’t quite realised what I was getting myself into here! I thought it was aimed at those with a more than generous tummy, rather than a problem only encountered by ladies.
The very best of luck to those seeking a solution. 
I just had this very problem before I got my current show coat. The first one I had, I spent $150 on and I lost all but one button by the time I was done with the 3 classes I was riding in!
I ended up trying the Horse Crescendo Haven jacket and am absolutely thrilled! The construction and fit is amazing! And it really moves with you. Have not lost a button yet. It has the look and feel of a custom tailored jacket!
I got mine for $99 at The Connected Rider.
http://www.theconnectedrider.com/?tap_a=9643-134fe7&tap_s=54629-678c64?
So beyond worth the price!
[QUOTE=Tanyanoel;8896648]
Thanks, I will check into adding a zipper that seems like a very good idea. The jacket fits lovely, I did what others have suggested already and bought a coat that has ample room in the chest area and have had it tailored to fit everywhere else, so it is not in any way snug across the chest it seems to be more of an issue of the, er, the chest movement and a big moving horse
I tend to lose a button perhaps every other show, although the first show I lost 2![/QUOTE]
Zippers can break under strain (ask me how I know. 
)
[QUOTE=Tanyanoel;8897317]
Thanks folks, great suggestions all, and I am already an Enell Bra wearer, which is helpful to great extreme but alas, the girls still move just enough to upset the occasional button. Perhaps I need to double up the Enell :)[/QUOTE]
I’ve heard that double undergarment support can help.
I don’t really need much so just wear a vest that’s held together with heavy duty velcro so that I can rip it loose the second I don’t need it, anymore. You might be able to make use of a vest layer too, for extra support, and velcro could also be of help because it’s easy to re-attach on the fly.
