Its not really replaceable. Well, it is and it isn't. The chassis that the fan sits in is custom made to fit in t he eMac, but the fan is just a standard 120mm fan. You would have to cut out the eMac's fan from its custom chassis, buy a new fan, cut the new fan out of its chassis, and then glue the new fan into the eMac's chassis. How do I know all this? I performed it all myself.
However, there was one major problem. The fan I used was not shielded, and so when I turned the computer on, the entire display on the eMac started wobbling and shaking do to the rotation of the fan (which is only like 2 inches away from the CRT's neck). In the end I was forced to replace back the original eMac fan. The sad part is, the new fan (a Nexus 120mm fan) was so quiet, it was impossible to hear!
If you want to keep it simple, you can either
1.) Buy a new eMac fan from an Apple reseller. Be warned, they charge you nearly $100!
2.) Buy a Zalman Fanmate to lower the voltage to the fan, thus slowing it down. This is the setup I have currently. It doesn't get rid of the noise, but it does help quiet it down.