I would say that finding a new original is as close to impossible as there is.
Used ones are generally highly corroded and the engineering number is usually 1/2 under the hose when installed so you can't really read it anyway.
New accurate reproductions were supposed to have been finished in March of this year but with the recent passing of Chris Brown, the person making them, that candle light has dimmed and is probably completely extinguished right now?
The next best step is to get the Scott Drake reproduction and machine off the casting for the '68 boss. It will at least appear to be the right one to a non-prying eye.
It will have the C8 engineering number on it and a SD mint mark as well but we don't live in a perfect world now do we?
It's also cast of a better alloy then the originals and won't crumble like wood eaten by termites like the originals did.
They were used more on 67 390 trucks then anything else but by now even if you found one still in use, either you will need to buy the truck to get it or it might crumble as you try to remove it?
The C7's are a rare piece and one in nice shape, even rarer.
Here is what I did with the Scott Drake housing. The boss where the 68 needs to mount it's vacuum switch has been machined off and you need to look closely to see the C8 engineering number.