Which coin battery should I buy?

Hello: I need to replace the PRAM battery of my iMac, which is a 24-inch (EMC 2134, Core 2 duo, 2.66 Ghz, model A1225). I need to have the battery ready for inserting, before taking the screen, etc., off the machine. Therefore I shall need to buy it with confidence that what I buy will be the correct battery. I assume that the battery will be of the "coin" type, but there are so many that I do not know what the identification number of the battery would be. Can you advise me?

With thanks in anticipation,

S.G.