Lebkuchen Schmidt, as far as I'm aware, only sell their own Lebkuchen. Their permanent store is by the
Viktualienmarkt, but they also have various other seasonal stores, including the one at Max Weber Platz that llees mentions. See their
website for a full list.
I don't know where you would find Haeberlein-Metzger Lebkuchen; it's not a brand I've come across.
Karstadt would be a good bet though. Otherwise bakeries usually produce their own Lebkuchen, which is generally fresher than the prepacked stuff, although at around €2 each also more expensive.