Purchase Great Prophets with Faith

I know that in the early game, you cannot "manually" purchase a Great Prophet with fait (though the game will do this "automatically" for you). I also know that at "some point", the ability to manually purchase a Great Prophet with fait becomes available. My question is... when? What technology/social policy/wonder/era etc... "unlocks" this?

The reason I ask is that when I build the Mosque of Djenne, to give my prophets/priests that extra use, I don't always build it in my capitol (either I have a higher production city or my capitol is otherwise busy), but the "automatic" Great Prophet *always* gets produce in the capitol. Or is there a way around that last part?

Industrial Era, I think.

If your Holy City is NOT your capital, Great Prophets will spawn there instead, I do believe.

If I remember correctly since BNW the mosque of djenne bonus is applied to missionaries only and no longer on prophets too.

At Industrial Era you are unlocking the ability to purchase Great People faith, depending on which Policy you have completed. Prophets are by default always available.

Tradition gives Engineer
Honor gives Generals
Aestethics gives GWAMs
Commerce gives Merchants
Exploration gives Admirals
Rationalism gives Scientists

Liberty allows you to choose a single any great person upon completion
Piety grants you a single Prophet upon completion
and Patronage grants you random people with allied City States.

So to answer your question.. Industrial Era is when you can purchase Great People with Faith.

Thank you all. The answer I was looking for is "Industrial Era".

And, for the record, the Great Prophet *does* still get the boost from the Mosque of Djenee (so, five uses). Also, my Holy City was (and usually is) my capitol; but I build the mosque in another city.