If I want to create a map about a region outside of the US, where can I find reliable sources of GIS data?
Depends on the country.
For the most part depending on the level of detail you want you may need to acquire it country by country.
You can get a lot of data direct from ESRI simply by using the Add Data from ArcGIS Online in an MXD session.
you can search for Nigeria Roads for example and get several hits.
For the most part you can simply use ESRI Basemaps.
I wanted to make a detailed map of Paris and London for a trip I took some years ago.
Finding data for Paris and France was very easy. I just used online translation to determine what field names meant. I wanted the street names in French because I would then be able to use street signs.
Getting data for London was very problematical. Open data was locked down tighter than a curling puck. I actually contacted a professor as the University of Edinburough and discussed with him what I was doing and he gave me the data.;
Checkout this Story Map that lists ArcGIS Hub deployments around the world.
Hubs Around the World
Some may have data you can use.
Hope this helps,
That's pretty slick and helpful
Retrieving data ...