It's hardware, you can't download it. Drivers are download-able software to control the graphics hardware.
Unfortunately, you'll find you need strong graphics hardware to power GC emulation. I'm talking NVidia GeForce 6xxx-8xxx power. Can someone recommend good hardware for GC emulation that isn't too expensive?
Also, before purchasing a graphics card, make note of what it utilizes to connect to the motherboard. PCI Express is /NOT/ PCI, and AGP, PCI, and PCIe are all of the different kinds you will come across. Also, if you do not know how to install graphics hardware, you may want to look it up or have it installed professionally.