This now-unusual ladies' accessory was once standard issue for the sidesaddle riding habit, back in the latter half of the 19th century.
In more recent times, divas like Baker and Dietrich wore toppers as the crowning touch to a tuxedo, whose ultra-tight tailoring ensured that there was no mistaking the lady within.
Most recently, Dita von Teese has adopted the top hat, pitched low on the forehead.
To my eye, ladies' top hats look their best with a medium-high crown and a short veil. Keyword "side saddle habit" and you may come up with a gorgeous version.
Finding the right place to wear it is actually trickier than sourcing the hat. One option: a gig, or better, a music festival (as long as you don't block too much of the view). Another: on a gorgeous autumn walk in a fashionable city park, horse optional.
(photo at top sourced from Corsets and Crinolines).