How do metal ions acting as enzyme cofactors "find" their respective enzymes?
The metal ions are transported across cell membranes by various mecanisms like channels, pumps or extracellular receptors for carrier protein.

The loading step mostly happens in the cell. Once transported inside, some ions just diffuse into their binding site on the protein; others, like iron, need a scaffold.

Some essential enzymes are metalloproteins, for example most ATP-using enzymes need magnesium; and the respiratory chain needs iron. Thus most cells will have transporters for these metal ions.