You can alsways get them from Asus, but given that you know the board, you also know what hardware it has, so if you want to check for updates, you'll find them on the Realtek site. Assuming you know your hardware (which you should, since you can go to Asus's website and see it, including model numbers of the Realtek parts), you can easily find them from Realtek, too.
In some cases, the base Realtek driver
doesn't include all apps or configs that the mobo makers put on their boards, so for audio, I'd stick to getting the driver from Asus. For the NIC, the Realtek drivers
are fine as well.