Where is it possible to get a very good 'truffe sous la cendre'?
It's near Strasbourg seen from Singapore maybe, but it's over an hour from there.
It's a very rare request. If it's important to you, I would indeed email truffle specialists with said request, because there is no warranty they'll have it on the menu on any given day even if they sometimes do. Bruno in Lorgues, Rostang in Paris jump to mind.
That said, if l'Auberge de l'Ill does it, why look further? Sure anyone can say they'll cook it for you, but already almost no one actually can do anything decent with a truffle, so imagine with such a delicate recipe.