Share your insights or perspective on the financial matter presented in the input.

Where do countries / national governments borrow money from? 
The answers provided so far as good and informative, but I just thought I'd add one small point... There are super-national organisations that commonly lend to governments, in particular those in the developing the world. The World Bank and IMF (International Monetary Fund) are the two primary ones. Also quite notably, the Greek economy was bailed out only this year by the EMF (European Monetary Fund) spearheaded by Germany - this is a rare occurrence however and was done mainly because Greece was a relatively developed country and others had an obligation to assist it as an EU member state.