Which countries / regions are iTunes / Apple vouchers available for? Where can I find out in which countries iTunes vouchers are available for purchase?
Unless someone has assembled a Wikipedia entry to track this, you would need to read the terms and conditions for each country. 

The website is optimized for people to find out terms in one region at the moment. 


https://www.apple.com/legal/internet-services/


Back in the early days, 2012, there was a matrix for this, but as the store has grown to 150+ countries and services, OS, features have made maintaining that a multi dimensional chess board of trackers.