... I wouldn't be shy to get the first cheap 5V module which already lies in the drawer and try it with a few mods as below. This is so because most HD44780-clones are wide supply voltage CMOS, capable of running at both 3V and 5V.

The mods are:

1. the controller's oscillator - there's a single resistor on the board which determines the oscillator's frequency. At 3V a different value resistor should be used to achieve the same speed of the controller; but IIRC the oscillator will run with the "5V value resistor" too, only a bit slower, which can be compensated for in software (this is what I did in my experiments - in fact, I simply checked that I already have the delays in initialisation long enough and the rest was driven by the status bit, so controller-speed-agnostic)

2. you'll need a negative voltage for the display. That's pin 3, where in the 5V system the pot is connected, usually turned down so that there's around 1V. As the display works against the + terminal, in the 3V system you'll need some -1V..-2V there. A single charge pump will suffice; you can do it either from a toggling processor pin and a diode doubler (inneficient but works); or using a dedicated IC like an ICL7660 clone, or steal it from other parts of your circuit e.g. if you have something like a MAX3232 around.

Some of the modules' PCBs are designed with this in mind, see the LCD dissected on my website (under curiosity).

3. backlight, and this is the biggest PITA. Usually the backlight are two LEDs in series so 3V is not enough to light them up even if you decrease the series resistor. The series connection is usually made on the miniature PCB holding the LEDs against the edge of the backlight waveguide (again see the dissected LCD), so there's no simple remedy for that there. You might be lucky to have a module where the series connection is made on the PCB, though, and then it might be easy to mod it.

On the other hand, backlight is usually completely isolated electrically from the rest, so you can use any higher-voltage source which is at hand. You can also use the same negative source than for the LCD supply, but the invertor must be capable of delivering the relatively high current the LEDs draw.

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If this would be for production, I wouldn't be shy to design these mods into the circuit if it would make sense price-wise (the 3V LCDs sometimes tend to be more expensive on grounds of lower volume usage) and/or the LCD is unique dimension-wise.

Of course, Michael's remarks as above hold too.

JW