The money will come from the same place money comes from now: banks. The value backing that money will come from the same place value comes from now: energy applied to capital. Given that the whole reason money and capital even came to exist in the first place is that humans have some compulsive drive to do relatively complex things once their basic, life sustaining needs are met, there's very little rational reason to be worried about less value being produced in a post UBI world.

One very good reason to keep (not necessarily in their exact current forms, mind you) the current bureaucracies involved in housing, health care, etc, is that that bureaucracy factors out the energy involved in obtaining those life sustaining societal structures. Certainly make the bureaucracy more efficient, but entirely eliminating it makes the society as a whole less efficient.