The black cat can be found on the not-Nakatomi Plaza levels. When you get to the foyer areas with lifts and fountains, boost yourself all the way to the top (may need some hose upgrades) and hopefully he'll be on one of the columns or lights hanging from the ceiling.
Unfortunately, since the levels are all randomly generated, there's no way of giving a precise location or method. It will just be a case of checking each time you reach one, if necessary quitting and reloading until you get the correct level type.