Welcome to iRV2.
Most radiator shops can fabricate a heater core if they have the original in hand for the measurements.
Can you see any indication of a brand name anywhere on the heater/A/C housing? Evans, Acme and Denso are the common manufacturers.
Any coach parts in particular ? Windows ? Doors ? Trim items ? Appliances?
Just post what you need , many members will be able to direct you to a source.
EDIT: I see you're here in B.C. there is an RV re-cycle shop in Lumby, don't know if there is any place closer to Williams Lake.