The majority of the economic and demographic data used in the making of the forecasts is widely available by institutions such as the CIA, IMF, UN, USG, etc. You can see the most relevant data at every single country's page. There is a tiny part of data coming from a variety of shadow sources such as Internet gurus, unsigned reports and others. But all these sources are from the internet and are of public domain for at least a minority.
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https://archive.is/kMWnbPopulation changes highlights:
Australia Down to 15,196,600 from 23,230,000
US down to 99,553,100 from 326,620,000
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