Use a LM317 voltage regulator. It's a very easy circuit to build and you can set the voltage to a very precise 7.5V.
Rainbow Kits has a kit to make this simple voltage regulator for not much $$.
The LM317 can be configured as a voltage regulator OR a current regulator.
I recommend against using a voltage divider using 2 resistors because if your supply voltage changes, you output voltage changes as well. Voltage divider is unregulated circuit.
Above is a my LM317 voltage AND current power supply for powering laser diodes. For a scanner, you would not need the 2nd current limiting LM317.
Another option I would do myself (but not recommend it to anyone else) is to simply use a LM7808 - 8V regulator. It's 0.5V higher than the scanner calls for, but scanners have their own built in voltage regulation circuits.