How do I obtain more spies?

The latest expansion to Civilaztion V added spies to the game.

These spies can help in a multitude of ways. The most important, in my opinion, being the ability to prevent enemy spies from stealing your technology. The main problem I have with this is that my single spy is struggling to prevent this theft, even with the Constabulary in place. The only other solution is to place additional spies in my high priority cities.

How can I get more spies?

The Gamespot game guide would seem to agree with @dbemerlin

• Each Civilization obtains a spy when any Civilization enters the renaissance era, with Britain gaining an extra spy
• You obtain new spies upon entering a new era.
• When a Civilization builds the National Intelligence Agency Wonder that civilization will also gain a spy

http://www.gamespot.com/features/civilization-5-gods-and-kings-walkthrough-6384284/?page=4