Or invent them! Many isekai reborn-in-a-game story lines focus on the large technical changes of society that the MC can bring with knowledge from our world. But true societal change can come from everyday items that the population actually needs.
Should bring in some decent profit as well, which will be needed in any type of game.
Well, "breast bags" were a thing in the middle ages, but just looking at them... I guess it's better than nothing? Cotton (or similar materiel) would be the deciding factor of success.
Nylon really is a miracle.
Although... just the concept of an underwire bra would probably make you a pretty rich person pretty darn fast; custom-made undergarments for rich ladies, made with the latest (underwire) technology, target the well-off Noble ladies over 30, then the highest-ranking Noble daughter's mother specifically, allowing her debut to boost business further...
Hey, don't use this as an AI prompt without crediting me, okay? Seriously.