Just because a picture was painted at a particular time does not necessarily mean that the dress is contemporary or realistic. Finding Historical Images Historical images are among the most common. You'll find them everywhere. Look at both general history and subject-specific sites. Or, for a picture of an early computer, try a site that deals with the history of computing or the history of science and technology.
News sources can be helpful for historical photos, and the stock houses surely are.
Don't overlook the nostalgia areas of the consumer services, which tend to be diverse and surprising in their coverage. Historical photographs only take you back as far as the mid-19th century. For pictures of events before that, you need to look for drawings, paintings, and other non-photographic renderings. Art history sites may help you, depending on your topic. Look at photojournalism sites on the World Wide Web.