We're already paying those people them money via welfare, so we've already figured out where that part of the money will come from.
The last category of people are those who aren't working (aren't paying tax) and who aren't currently claiming benefits. The money for these people will come from the money saved by reducing administration overhead on the welfare system.
If that's still not enough money to cover it... then yes, you're going to have to start working out where it comes from. But the bulk of the money is in fact already available or easy to find a source for, and any shortfall will be nothing like the $10,000 * 300 million that the article thinks you'll need to find.