I looked at websites eBay, J&P Cycles, David Silver Spares, CMSNL, Partzilla, and 4into1.
I searched eBay three different ways :
once with "1978 Honda CB400 exhaust muffler"
once with "Honda 18430-413-003" and
once with "Honda 18430-413-013"
also putting a check in the checkbox for eBay to also search descriptions along with listing titles
It appears the mufflers on your '78 CB400 are only compatible with '78 and '79 mufflers like "L.D.R."-Jim mentioned above.
There is 1 used left-side muffler on eBay in California that looks pitted and worse than the muffler on your '78 showing in your photo above.
There is also 1 new (NewOldStock) left-side muffler on eBay in England that I will send you the link to privately.
J&P Cycles shows to have 2 styles of new EMGO (aftermarket) mufflers on their website.
David Silver Spares and CMSNL appear to have zero in stock but both will take your contact info for future IF they acquire any.
CMSNL has in stock new aftermarket sets of head pipes if that is what you meant instead of tailpipe mufflers.
The aftermarket exhaust manufacturers that I know of are:
MAC, Delkovic, EMGO, Vance & Hines, Kerker, Bassani, and Yoshimura
Another option would be to cut out the rotten rusted section of yours and weld in new metal and maybe then have the repaired pipe or muffler re-chromed or if the rot section is small enough, maybe wrap it closed with special exhaust pipe wrap.
I've also seen a BringATrailer Honda seller in Wisconsin has who sold several used '70s/'80s Honda bikes with "repaired" mufflers that may be worth contacting to learn how they repaired their rust-rotted mufflers on bikes they've sold or serviced.
Check your private messages here for the link to the NOS muffler in England that I am going to send now.