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.htm
I 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.