Many OEM manufacturers such as Gateway and Dell provide a single Firewire port as a part of their systems, run by a controller on the motherboard. Some retail motherboard manufacturers such as Aopen (AK73-1394) and Abit (IT7, AT7) are also providing Firewire functionality onboard. Creative's Audigy series also have one port built in.
The other way to gain Firewire is to add a PCI card to your system. There are hundreds of manufacturers that make Firewire cards at various pricing levels; however the main thing that varies is the software bundle. If all you want to do is networking, then all you need to get is an OEM card, available for under A$100. You can search for the best price on Firewire cards at Pricegrabber.