As said a firewire cable can be bought from just about anywhere that sells camcorders. The quality of a cable does not really come into it as it is digital and either works or doesn't!
If your PC already has a firewire port then you will probably need a 4 pin to 6 pin cable as the camcorder will need a 4 pin plug and most PC's will have a 6 pin connection on them, but some might also/instead have a 4 pin connector so its best to check (RTFM

).
If your PC does not already have a firewire port then you can get a PCI card for around £15 and this nearly always comes with a cable supplied.
Mark.