I have chosen OA79 which provides 0.47V and changed design slightly. There was some time limitations and I doubt whether Schottky diodes will be available in local stores!
Try the very common 1N5817 to 1N5819 series schottky diodes. They present maximum forward voltage of 0.34V at 0.1 A forward current. Typical curve present 0.3V @ 0.15A
Connect a 20 ohm resistor in series with the diode and a 3.3V voltage source. The forward voltage should be around 0.3V over the diode.
Cause you didn't tell any requirements, it's impossible to give an exact suggestion. For reference voltage, I would used a divider with a buffer. For power supply, an usual adjustable regulator with additional circuitry to lower the output voltage could be used.