The USGS Map Locator is the place to go for topographical maps of the United States. Even though the website says "The USGS Store" there are free downloads that can be printed at the Alkek One Print Shop for Texas State University students, faculty, and staff.
You can search the Map Locator by address, location, or topographical map name. Clicking on the resulting "pink pushpin" reveals the various scales available for a given location, along with (some) map previews and download options.
The Sanborn Map Collection is a series of fire insurance maps created from 1867 through 1970. Although they were published by a commercial entity, copies of the maps were sent to the Library of Congress for copyright deposit. The resulting collection documents the development of American cities and towns from the mid-nineteenth century through the mid-twentieth century.
While the Library of Congress is digitizing the collection, it is a slow process ... only 4000 of the 600,000+ color images are available online. Fortunately, Texas State University students, faculty, and staff can access a digitized black-and-white version of the Texas maps in the collection from ProQuest.
As an added bonus, Government Information Library Assistant Gaye Wood has compiled composite views of San Marcos using Sanborn maps from from 1906, 1922, and 1930 (with permission from ProQuest). A composite from 1912 is in progress.