The driver from Canon does support 10.8. It will be the same driver that Apple has available via Software Update, which by the sound of your post, your Mac is trying to locate the driver from there instead of using the driver that is already installed.
One action you could take is to open Finder and navigate to HD > Library > Printers > Canon. Here you will see the BJPrinters folder. You can trash the contents of this folder and then restart your Mac. After logging back in, try to add the printer again and see what happens.