The President can only divert the Military Construction money to the Wall because of a particular Law - The National Emergencies Act of 1976.
That Law specifically authorizes those specific funds to be spent on construction at the President’s discretion in an Emergency (and empowers the President to declare that emergency).
Funds must be both legally appropriated, and legally authorized by Congress, to be legally spent.
In this case, one law appropriated the money (NDAA), while another specifically authorizes its use - but it legally both appropriated and authorized.
There is not much more in the Federal budget that could be diverted, beyond what he has already (very resourcefully) found.