Two years ago, I asked my mechanic about it. He said I could order an OEM, new, from Honda... $1,800. If you really have to have OEM, check with your local Honda/independent shop, to see if if Honda still has them in stock.
The problem with any originals, is that water is a byproduct of combustion, and it gets blown out the exhaust. This becomes a problem when the bike is not ridden long enough to evaporate all of the water out of the exhaust system, which leads to rusting... Mine was fine, when I bought it, back in 2006, but now I have the lower right muffler with a hole, rusted from the inside. It is extremely rare that I rode it less than 30 miles. Time takes its toll.
I did find a forum where a fellow with a '79 K, had used slip-on replacements ($89/pair), attached to the OEM headers. He maintained the 4-into-4, and it looked fantastic. Only issue was they were painfully loud: internal baffles came with, but they were still too loud (metal only). I found a site that details how to quiet mufflers down, by inserting a series of half-washers, accompanied by wrapping fiberglass stove pipe rope around the original baffle. Anyway, it is untested, by me. The fellow who wrote the article has oscilloscope readings, showing an at least semi-scientific analysis.
Whatever you decide, best of luck! Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII