I would sugest that you call Beaver Coach Sales in Bend OR. for the part. They will probably either have a heater core in stock or they will have access to the manufacturer of the heater core to obtain a new replacement. However if you do not have the time or money to replace the heater core at this time, by-passing the heater core (by hooking the hoses together) will work very well with the only down side being the loss of dash heater function.
Gerald