In Civ4, Great Prophets - or, more correctly, Great People - are spawned by having more specialists. Having more
food = more specialists. Food is power in Civ4 in terms of getting Great People, so if you have say, 10 surplus food, then that's quite a few specialists you can use. If you like using Great People, use a philosophical civ (like Arabia - they may very well be the "New Babylon" this game). Philosophical civs give +100% Great Person growth. There's also a few wonders and civis to help you in your cause:
Parthenon - Requires Polytheism, obsolete with Chemistry. (+50% birth rate in
all cities, as well as +2 Great Artist points). That's 150% right there with a Philosophical civ.
Heroic Epic - 100% Birth Rate. Requires Barracks. This is a national wonder, so you only get two per city, so plan carefully.
National Epic - 100% Birth Rate. Requires Library. This is a national wonder, so you only get two per city, so plan carefully.
Pacifism - Requires Philosophy.
100% Birth Rate in all cities with the state religion.
Caste System - Unlimited Artist (great for culture bombs), Scientists (great for techs, or academies), and Merchants (great for extra gold).
Let's add that up!
Philosophical (100%) + Parthenon (50%) + Heroic Epic (100%) + National Epic (100%) + Pacifism (100%) = 450% Great Person rate!
Toss in Caste System, and you get unlimited specialists. One specialist will yield not 3, but 13.5 GP points! Do note that the Parthenon goes obsolete with Chemistry, and you can only have 2 national wonders per city, and it might not always be ideal to stay in Pacifism (you might want therocracy for the extra experience points).
If you can make that city your wonder city, most wonders give you +2 great person points. Let's say you have a typical wonder city with the above settings. It's got 4 wonders, and your two epics (it's a city other than your capital, as a palace equals one national wonder), giving you 10 GP points. It's got a lot of food, so you give it 5 scientists (15 GP points). You now have 25 base points. That's now
113 GP points total!!!. This is pretty potent stuff.

Along with the culture bomb, and culture being added to city defense, culture is now a true weapon.