Most LCD require very little current on the negative voltage bias supplies. These are usually supplied by charge pump ICs. The produce a negative voltage by using capacitors and internal switches. First the cap is charged to your positive rail, then internal switches reverse the leads on the cap and connect the positive to ground.
Most KS0108 128x64 GLCD's already have the LCD display voltage converter on-board. It's the Vee pin. Run this voltage through a small pot to the Vo pin and you have all you need.