Most of the more obscure models will have been scrapped, but there are still quite a few 600hp-1500hp yard switchers and road switchers in regular use even today, in branch line roles and industrial switching service. You will also be able to find a sizeable number of GP30/GP35/GP38/GP40 general purpose road units as well.
For what we are talking, there will be plenty available. There are plenty of these units in the large BNSF and CN/CP yards in the Chicago area, some still wearing their pre-1987 Milwaukee Road paint.
I'd recommend the rail classifieds at
www.railswap.org -You can see a number of these units for sale as well as pictures of the other types of equipment needed.
Occaisionally you even see stuff for sale abroad-