At HH cup the number of swimsuits that genuinely work is small, and most of what is sold as supportive simply is not at this size. What does work uses long channelled underwires, firm cup fabric and a band engineered to carry the weight.
What to look for at HH cup
Depth first: the cup has to contain the full bust without the top edge cutting across. Then the band — it should feel firm on the loosest hook when new, so it still supports once the elastic has relaxed. Wide, adjustable straps set close to the neck stay put best.
Brands
Elomi Swim is the principal range at HH cup. See all all HH cup swimwear in your size, or the full one-piece swimsuit range.