where are scanner drivers stored?

Mostly they are part of support drivers for printers and scanners if it's a multi-function device. Otherwise, they come from the scanner maker's support site where you can download the drivers. Check Apple's OS X- Printer and scanner software available for download. If OS X has a compatible driver for your scanner, the once the printer and/or scanner is connected and turned on you should be prompted to install it when you open Printing & Scanning preferences and click on the Add [+] button.

If your scanner isn't supported already, then I highly recommend using VueScan if it supports your scanner.

It depends on the make and model.

Most scanner drivers are installed in /Library/Image Capture/Devices for ICA drivers and /Library/Image Capture/TWAIN Data Source for TWAIN supported scanners.

I've also seen files installed in a manufacturers folder in the Applications folder.