You get the rod from the same people, generally you buy a rod specific for the props you want to balance. For me it was the rod for self tightening props.
Basically it's a really accurately balanced spindle to put your prop on, so that if one blade is heavier, it will roll that blade to the bottom.
Getting the rod is really all you'd theoretically need, but if you're going to go to the trouble of balancing props then you may as well also get the ultra low friction device that holds the rod so that it will allow the prop to rotate if any side is even slightly out.
All my props except my 3 blade ones are genuine DJI and all the genuine ones were a tiny bit out. not enough to be the slightest problem. I doubt you'd even notice with how little they were out, but they did all have one side that was the tiniest bit heavier than the other. I balanced them anyway just for the heck of it till neither side would move in whatever position you put them.
The 3 blade props were a nightmare!!! Balancing them was such a PITA that I decided since I'm never likely to fly with them, that I'll just write them off as a bad mistake.