The voltage depends on your diode's voltage at the current you intend to run. For a PHR, I think they're typically slightly above 5V at somewhere around 100mA and you'll need >2.25V over that to input to the driver. So, two 3.7V rechargeable lithium batteries is typically sufficient. If you're going much above that, you may need to add a third battery or the current will start to drop as it goes below the minimum. It will continue to put out current below the minimum voltage, but it will drop off as the voltage gets lower.
You won't be able to find rechargeable lithiums anywhere locally (unless some kind of specialty shop has them). But, there are some US places that sell the batteries. I think someone on here offers RCR123A... they just posted something the B/S/T forum about it.