Getting 9V and 400-500mA out of a 24V power supply
As of now, I only have access to resistors for help. If there is a strong reason NOT to use just resistors, let me know! (I'm not an EE).


Strong reason. Wasted energy, fire hazard, and voltage will vary based on load. You are wasting (24V - 9V) * 0.5A = 7.5 Watts of energy in heat in those resistors! Horrible.

A small switching regulator module will work nicely. Heck, a common car usb charger, with a few resistor changes and maybe a new input capacitor (for greater than 24V), will do what you want. Most dollar store to 5/10 dollar chargers at a local corner store use a MC34063 regulator IC. Takes up to 40v input, adjustable output with a resistor pair, and can do 650~750mA without any other changes.



Even better, USE 5V NOT 9V! Giving the Arduino 9V will make it go through the regulator on board, wasting another 4V * 0.5A = 2W of energy in heat. Instead, use the 5v out of the usb charger to the usb connector of the Arduino (You would need to make a usb cable). Probably only need to change the input capacitor at that point.