The University of Missouri Libraries are developing a great resource for genealogists and other researchers. Among other images in its Digital Library are great collections of maps. I will mention only two of these in this post.
Sanborn Fire Insurance Map Company published detailed maps of cities and towns to enable insurance companies to assess risk. These are great for finding buildings during the last half of the 19th Century and the first half of the 20th Century. If your Missouri ancestors lived in one of the almost 400 towns covered by this series, you may be able to find them in one or more of these maps.
If on the other hand, your ancestor lived on a farm, you will want to check out the Plat Book collection that covers almost all the entire state of Missouri.These were published in 1930 so the locations shown were ownership of land prior to that time.
These are just two collections contained in this impressive Digital Library. You also will want to explore the other treasures that can be found here.