I am looking for a shapefile with the country boundaries as they were when Czechoslovakia existed as a country.
Is there any online open source resource where I can find historical country boundary shapefiles?
At the world level, the historical-basemaps project offers boundaries for many countries/cultural areas from various periods between 123000 BC to 1994, in geojson format. There are world maps for 1920, 1938, 1945, and 1960, where Czechoslovakia borders are available.
Note that the project has the following caveat:
This historical boundaries project aims at providing ready-to-use base maps for mapping historical data. It is work in progress: verify the maps by comparison to other sources before using in academic work. If you see errors, report them in the "issues" section.
If you use the maps, think about giving back by contributing.