There is a Tiara Rule where you cannot just purchase one immediately, you need to inform your family first and have discussions or prior planning before purchasing a genuine diamond or gemstone tiara.
You can win a real tiara from Beauty Pageants. You are allowed to wear the advertised tiara for official appearances during the year. Thereafter, they will gift you a much smaller version as a keepsake.
Mikimoto makes pearl tiaras all the time, in fact there is a Mikimoto Phoenix Pearl Tiara in the Miss Universe Pageant. Here is a Hello Kitty tiara released in 2014, with matching Hello Kitty pearl jewellery. The latest 2019 tiara "La Flora de Mikimoto" pic below.
European Jewellery House Garrard "by royal appointment" makes tiaras for European royalty. Princess Charlene's convertible tiara can be worn as a necklace as well. Smaller tiaras can be detached into a bracelet or a pendant.