Good sources; UNDP, World Bank, OECD, CIA World Factbook...
This is my go-to list: https://data.worldbank.org/indicator
Princeton also collected many sites here: https://libguides.princeton.edu/internationalecon/major
Here are some good sites used by The Guardian; https://www.theguardian.com/global-development-professionals-network/2016/mar/16/the-top-10-sources-of-data-for-international-development-research