"Curve Focused Since 1921
Bras for every body in band sizes 30-58, cup sizes B-K." 
" More Sizes Are in the Works
Oh, no! We don't have your size right now, but we're always adding more to our collection. Sign up below to be the first to know when your size becomes available."
Yeah that's what I thought. 
It's always the same spread, like 34-42 in any cup size you want, or any band size but only in a D or DD. It's astonishing that we have not jointly raised hell to demand this industry create everything custom.
Actually, it's weird that we don't have more "give us your dimensions and we'll laser-cut and sew you something" businesses by now. The only place I have seen doing that sells jeans.
I think the thing pissing me off most in the lingerie access world today is that I've never seen shapewear cut for larger cup sizes. If you want a bra that fits, but you really want kind of a midsection shaper, maybe a full bodysuit affair, something you can just throw on that handles everything at once?
That tops out at a DD. I saw a 48 today. I think 48DD is actually the exact size a lot of people *think* they wear because it's often the largest they can try on and no, it's not cutting into their flesh, but they are actually more like a 42I or a 44G or something... and you can't get those. Well, who the hell do they think would appreciate some extra support?
This is also the point where I should point out that you absolutely can have someone draft a custom bra pattern for you and sew a standard, well-constructed underwire bra for you from scratch. Such services do exist. The prices I've seen range from $450-$800 per bra. At that point, without a very specific reason for the purchase, I think most of us would say that service effectively does not exist. But it does make me wonder what we could do if we set people with cutting gizmos and such up with people who are willing to iterate on cut patterns and get theirs right, open sourcing their custom sizes for others with similar shapes to try to adjust. As it stands, I can't simply go to Jo-Ann's and buy a bra pattern that could be sized up that far in any sensible way, but a homebrew set of "bra: DDD-G, H-J +custom band sizing" digital patterns which have gotten over the worst of the translation would give people a starting point, at least.
@JuliePercival this site is great, thanks!
@pmosetc try biggerbras.com. That’s where I end up buying almost all of mine. There or Nordstroms.