You need the original AirPort card from Apple, which is no longer in production and has a high street value:
http://www.memorysolutions.com/memorysolutions/usorapaicave.html
You can find cheaper prices with a little Googling. Make sure you get the AirPort card adaptor piece as well, as it's required for installation in your model iMac:
http://www.memorysolutions.com/memorysolutions/aicaad1.html
Be aware that the original AirPort cards such as these use an older 802.11b protocol, which only transfers data at 11mb/sec (as opposed to AirPort Extreme 802.11g, which can go in excess of 20+mb/sec), which makes for pretty slow file sharing with other wireless computers, but totally suitable for broadband internet use.