MCI parts use to, and maybe still does, buy their radiators from Diesel Radiator in Chicago.
About a year and a half ago I about a pair of 6K-1-5 (102 radiators) for about $1800 for the pair. You will have to modify the panel in front of the radiators to get them to fit - but it is not difficult. Craig Shepard shows how on his site
http://www.gumpydog.com/bus/index.htmI would not recommend buying used radiators since cooling is very marginal on MCIs. Make sure you have the large fans and small upper pulley to get the best air flow.
Ed Roelle
Flint, MI