There are two different types of LED power supply: constant current and constant voltage. You need the latter.
It must not have PWM or significant ripple on the output, but a stable voltage.
It must be capable of outputting enough power at the right voltage and current to power the power supply.
Why not simply buy a 300W constant voltage PSU? 24V is probably optimal, since the PSU can step up, as well as down and it's half the output voltage.
For future reference, ask specific questions and post in the beginner's section, if you're unsure, because you'll get an easier time.