depending on the wattage of your MH ballast, you may be able to use a 'conversion lamp' from here: HID-Lights - High Pressure Sodium Light Bulbs - 1000-Bulbs - The Light Bulb Superstore

I use a 'conversion MH lamp' with my 400-watt sodium ballast, it is working just fine, (I couldnt afford another ballast) ... :smokin: