The Sankei reproduced exhaust system comes from SixCenter Motoren, in the Netherlands. http://www.sixcenter.nl/nieuws/
In the US you can get them from Northeast Vintage Cycle http://www.hondanuts.com/home.html
However, you won't see them on their web page. They sell them on ebay, or you can call them at (716) 592-6382. I'm pretty sure that if you call them you will get a better deal than ebay.
They may be waiting on the next production run from Japan.
I have a set, and to me they look exactly like the original exhaust system for a '79, but I have not installed them yet, so I cannot say for certain that they are perfect. They are marked HM422ED, which I believe is a European exhaust system, but I don't know what differences there were between European and US models. Probably nothing noteworthy. For a '79, I think this may be as close to an OEM exhaust system as you will ever find. After all, they are made by the same company that made the originals for Honda.