I came across the Spanish museum Museo del traje’s website and figured out how to search their collection. They have lots of beautiful 18th century clothes. Click the picture above to go directly to the search engine.
If you know Spanish you won’t need any help, but if not, I wrote “indumentaria historica” (a term I found elsewhere on the site and seemed to mean something like historical clothing) in the “Búsqueda General” search field and 1700-1799 in “Datación”. You can’t write directly in the “Datación” field, you click the “Fechas/Periodos” button and write the first and last date you want in the two “Año” fields and then press “Aceptar”. Finally press the “Buscar” button.
That seemed to get all the 18th century clothes, but if you know Spanish and a better way to search let me know in the comments.