You can do a search for LOA or Lights of America. They have an 18w fixture that costs $25 or so. Many on this board say it is the best (budget) light for growing Caulerpa. That said, after two trips to both Lowes and Home Depot, I have yet to find the light. I have seen it online search "Lights of America refugium" on google.
On My tank. I have 2X20w NO's on my fuge. You want to get a 5500K or 6500K light for the best algae growth. LOA is 6500K. I leave my lights on 24/7 it prevents the Caulerpa from going asexual (releasing its absorbed garbage back into the tank) Leaving the lights on also prevents a PH spike at night.