One source of funding for the wall could come from disaster recovery funds.
The President has not declared an emergency, but if he does, they will be scrapping funds from wherever they can find them. As this article notes, it could come from disaster recovery funds already appropriated by Congress, Disaster relief, Pentagon: Where does Trump get the money for an emergency declaration?
The challenge then--should those funds be tapped for the wall, is getting them replaced by Congress. You might think it would be automatic, but these days nothing is automatic in Washington D.C.