That's a tough one considering the age of your bike.
Vance & Hines (maybe?)
I can't think of any more brands that would have made a 4-1 exhaust. You might could cross reference the stock header spacing and research what other bikes from that era had the same spacing to help with availability of your part search.
Ebay and these forums are the only place I can think of where someone might have what you're looking for. Only other place I can think of is a motorcycle salvage yard.