Stop digging through sites that go up to a 38C and call it "extended." Here are brands that actually carry your size — filtered by what you're looking for.
Most mainstream retailers carry a limited size range because producing extended sizes costs more in materials, requires separate fit testing, and was historically seen as a niche market. Specialty brands that serve extended sizes have existed for decades, but they were never well marketed or easy to find. That's changing — but slowly.
Many retailers offer a "plus size" section that's just their standard styles scaled up mathematically — without the additional engineering that larger cup sizes actually need. True extended size brands fit-test at every size range, use stronger fabrics and construction, and design specifically for larger busts rather than just scaling a B-cup pattern.