Year and model?
Majestic isn't showing any mufflers available for the 1990 Si, for example.
If Honda Canuck cannot find a muffler (I'd say that they're probably right), all they really need to do is get you a suitable replacement. A replacement for a fairly [crummy] OEM muffler is a [crummy] aftermarket one. They don't need to get one that you like, per se. I would, obviously, try to get the best aftermarket one out of them that I could, or failing that, offer to split the difference so I could get an aftermarket one I liked that would work.
If you're still stuck on OEM, you're right - the only option you're probably left with is calling individual dealerships up. I think that's a waste of time, but you seem to value the OEM muffler more than I. Have fun with that.