The Victorian Web site has a wealth of articles on life in the Victorian era. You could spend a very long time reading the material there.
The New York Public Library’s Digital Gallery has color plates of German military uniforms dating from 1740 – 1900. These include the uniforms of German states such as Anhalt, Bavaria, Saxony, Hamburg, Hessen, Prussia, Nurnburg, Saxe-Weimar, Wurtemburg, and Westphalia.
For a terrific collection of color plates of French military costume from the Gauls to 1806, visit The New York Public Library digital galleries.
The New York Public Library Images collection has an amazing collection of the uniforms of Russian armed forces from 960 – 1900. Most are in color. All are useful.
The New York Public Library’s digital holdings have an amazing collection of color plates of British uniforms from 1066 – 1900. These are great painting guides.