It comes from, surprise surprise, the cyclical economic pool.
The United States GDP per capita is 53,041.98 USD (2013). This is nearly $4000 per month. Assuming that approximately 33 percent of the American population would qualify for minimum basic income, it would constitute less than the country's annual military spending to utterly eliminate homelessness, poverty, starvation, lack of education and illiteracy, as well as drastically improving a nation's psychological health.
Constantly screaming "WHERE DOES IT COME FROM?!?!" is significantly less helpful and constructive than actually reading the f***ing articles and finding out where it comes from.