No antivirus software is needed as there are no (as in zero, none, nada, zip) viruses for OS X. If you insist though you can download ClamXav, simply Google it and you will find it. However it's really unnecessary for OS X, the ONLY reason someone could use it is if they download unknown files off the Net and then pass them onto others that own PCs. If an infected file is downloaded it will not affect OS X however it could affect the PCs. If that describes how you use your machine then perhaps ClamXav would be useful however if it doesn't there is really no reason to download it.