What sort of current draw does each draw at the 9V and how many do you plan to drive at once? The final current draw makes a world of difference.

Since you don't need an absolute 9V supply - the diode method would be by far easiest. But this will be a diode running at 100% duty cycle and a steady state (at 'TBD' level) current -- this has to be far less than the 600mA total or could be a huge temperature rise.

The linear regulator (LM2940) would also work and give a steady voltage at a level 9V. But, again, need to know current draw to make sure you aren't overloading. These will also dissipate a tremendous amount of heat and would need to have a pretty heft heat sink.

As the others mentioned - best bet would be to use the switching supply mentioned by SteveNZ. Since they are switchers - MUCH less heat. But since these are no-name Chinese assemblies, I wouldn't expect a tremendous life span out of them.

Yep, amazing how the Chinese can steal designs (why bother paying designers when you can just 'borrow' the completed design from somebody else?), pay slave labor wages, use inferior parts and still get subsidized by their government.

But that last ebay link. That's probably the easiest way for OP to do this. It appears to be nothing more than a rheostat.