You can download driver for Canon CanoScan LiDE 50 ScanGear CS-U 7.1.3.3 here, but DriversKit`s Team strongly recommend you to use Driver Updater Pro for finding latest version of this driver. You can download Driver Updater Pro here.
Also you can:
Search Canon CanoScan LiDE 50 ScanGear CS-U 7.1.3.3 driver at Google