Your best bet for Realtek drivers (assuming you are using the standard AC 97 codec) is searching for 'AC 97' on download.com. Alternatively go to Realtek's website and download the drivers there. If you are using Windows 2000/XP/2003, make sure you download the WDM drivers (these won't work on NT4.0/9x/ME). You can find drivers for these OS's by searching on download.com as well
Be aware though, my Realtek AC'97 was already pre-installed on my emachines (gateway) PC. When i decided to download the latest drivers, i was presented with all sorts of problems. For example, the sound would cut out every now and again when playing music in Realplayer.
Therefore, your best bet for drivers is on the original driver disk provided by your PC manufacturer. If you don't have the disk, you can normally download the drivers from your PC manufacturers website.