In the course of the 19th century, the validity of art as a source of costume study declines. At the beginning of the century, the Classicist movement again emphasised realism and a certain naturalness. Soon, however, art begins develop a number of movements that postulate different degrees of realism, discover deliberate abstraction and claim more artistic license than ever.
favours scenes of Greek and Roman antiquity or "historic" paintings
in a heroic style. Romanticism employs colours to evoke a certain mood: In one
painting by Caspar David Friedrich, the night sky appears brown, in another
of the same scene, it is violet. The subjects are landscapes, if not dreamscapes,
rather than people. The Preraffaelites go for historical or legendary subjects
painted in a realistic (but not authentic) style.
The Impressionists favour everyday subjects - in the case of Toulouse-Lautrec even lowlife such as whores -, so they should be very interesting. But unfortunately, they also favour a quick and rough stroke of the brush that effects a certain haziness and obscures detail. In their love of colours, especially white, they have no qualms about using the colours they like rather than those they see.
Art Nouveau, under the influence of Japanese wood prints, moves away from plasticity: every object is outlined in black and fabric folds symbolised by black lines. Shapes are adapted to follow lines perceived as graceful. While this aesthetic does not reflect reality, it nevertheless influenced all kinds of applied and decorative arts, including fashion. This is pobably the first time in history that art has a direct impact on clothing. The lines of clothing and especially ornaments try to follow the Art Nouveau aesthetic.
At the turn of the 20th century, movements that used abstraction to a greater or lesser extent (e.g. fauves, expressionism) had grown so strong that suitable paintings are rare finds. "High" art as found in museums and art books eschewed realism; whoever painted realistically was regarded as mererly painting commissioned mainstream, earning a place above some mantelpiece but not in a museum. (As though the "old masters" had ever painted for fun!) Sargent is one of the last who painted realistic portraits for money but was not barred from museums.
Fortunately, photography developed at the same time as realism in paintings declined, and fashion magazines became more widely available.
For realistic art that still contains enough detail, look for David, Vigée-Lebrun, Ingrès, the early Manet and Monet, and of course Sargent.