I am researching old maps of the world from the 1920s and 1930 for a project I am working on.
Two great resources I have found are:
http://www.hipkiss.org/data/maps.html – some maps here
http://www.oldmapsonline.org/ – map scans. But not as good quality, or as comprehensive (or well indexed) as this one
https://www.davidrumsey.com – excellent very high quality scans of maps through history that you can download.