I can imagine that model board only comes with two not three or more pci slots. One place that carries I/O expansion cards is seen at
http://www.startech.com/Product/Category.aspx?WCID=211&WCLID=40&MLID=7&menu=1&c=US
For simply being able to add devices other then video, sound, tv tuner pci type cards you can also opt to install an expansion card to add additional usb ports through the pci bus. If You plan on using various pci devices like those other then just a sound card instead of onboard and a tv card you want to reconsider a model that offers three or more pci slots.
Generally expansion adapter cards are good for temporary use and not really the best for long term application. Abit and DFI alike always seem to lag behind other brands like Asus, Gigabyte, and MSI in that regard.