It's been a real pity that the Imatex online collection hasn't been available for ages because they had some very pretty extant 18th century costumes in the collections. Now some of the objects are available on Colecciones en Red online collection. The page is in Spanish so if you don't understand the language it can be a little difficult to find things. However, after trying various options I've found a way to get around.
Try search words such as polonesa (polonaise), vestido (dress), Museo del Traje and robe and click 'buscar' to search. After getting on the list of the search results, click on the 'Ficha completa' for more information and to see a bigger image. Click 'Ampliar Imagen' under the image to enlarge. There's also advanced search 'Busqueda avadaza' available and it's not too hard to understand the search options, specially if you use Google translator. I know Google translator is usually a bummer but in simple cases such as this it helps you to get an idea about the texts on the page.
You can find some of the objects that are in the collection more easily on Museo del Traje's Visita Virtual page.
This is definitely not one of the best online collections I've seen but it's nice anyway. The large views of the images aren't that large, really, there aren't many views on the objects and you usually can't open the links on tab, which I find tedious because you have to click the 'more information' link and get back on the search result list page in order to click the next object for more information.
Happy searching! ;)