Just do it out of the military budget. This is squarely a national security issue.
As far as I know, Congress appropriates money in the defense budget. I don’t think Congress then has a say in how those funds are spent - i.e., do we build an aircraft carrier, a new military base, or a border wall, for example. So all they would have to do is move some money around.
The border wall fist into so many places in the budget, infra structure, national defense, drug/contraband interdict, human trafficking etc etc etc.