24v power supply to 24v and a Arduino
Using a switch-mode buck converter is the most energy-efficient method of doing what you want.  The reason that I mention energy efficiency is that if you used resistors as per the image that you linked to, you will need to get rid of the excess heat generated.  This amounts to (24V - 8V) * 0.5A = 8 Watts.

The dc buck converter that you linked to is also available on eBay for significantly less than Amazon's price.  In fact, it's available on eBay for less cost than purchasing suitable resistors from Digikey.