As with other images or texts, online maps can often be found with a general web search such as:
You may find additional maps by not limiting the domain, since many .com or other domains will offer maps. Be sure to verify your sources if you use "non-official" websites.
There are many online repositories of images, which are often created or hosted by universities, libraries or historical societies. Try a Google search using a complex string like the one below to focus your search for relevant images.
Many libraries, museums, and other cultural institutions are continuously working to digitize texts, audio & video files, maps, images, and other material to make them widely available via the Internet.
Because copyright law may limit the ability to digitize more "recent" material, these digital collections are especially useful for historical material. They also generally have many government publications, which are not protected by copyright and may therefore include very recent information.
The largest of these projects are linked here; browse their collections or try a keyword search and you may be surprised by what you find!