I also have this "problem," as I am currently about a 40J, give ot take -- I've yet to find something that really fits, have tried to do professional fittings, but even with this size the cross between the breasts still doesn't lie flat. Unless someone sees me braless, I get eyes rolled that I "really am" this size.
I've found that Just My Size has a catelog with larger sizes, and can be ordered from online or a mail-order catalog. I'm also planning on trying to do a reduction, but want to wait until I'm not going to have any more kids, both because of the risks of BF'ing after a reduction, and often pregnancy and BF'ing can change your size, either larger or smaller.
Another issue many haven't mentioned is that not only is it harder to find flattering bras, but it can be harder to find appropriate shirts, jackets, dresses, and any other clothing that would be worn on the upper body. There is little if any lingerie that wouldn't look like it was cut for a pre-teen on me.