The quote is actually a genuine one. In the original, it reads, "Für die vorzüglichste Frau wird diejenige gehalten, welche ihren Kindern den Vater, wenn er abgeht, zu ersetzen imstande wäre."

As far as the source goes, things are a little tricky, but not really a huge problem. It first shows up in Goethe's novel, "Wilhelm Meister's Years of Travel" (Wilhelm Meisters Wanderjahre), in the second version of the book. It appears in the chapter "From Makarie's Archive." (Makarie is a character in the novel.)

There are also posthumous compilations of Goethe's sayings, aphorisms, etc. One such standard title is "Maxims and Reflections," which has made its way into the Goethe canon, although different editions with that title may have slightly different contents. (I said it was tricky.) So that's another place you might run across the "excellent woman" quote.

Not sure where Margaret Fuller found the quote--maybe a Thomas Carlyle connection, but that's a hunch. As always, there is more to be said.

Sources (auf deutsch):

Gero von Wilpert: Goethe-Lexikon (Kroener, 1998)

Goethe: Wilhelm Meisters Wanderjahre (in complete edition, ed. Neumann and Dewitz, Deutsche Klassiker Verlag, 1989)