Ideally you'd get a 4' long fluorescent fixture (the 'right' bulbs only come 4') and put in Full Spectrum (high watt), or Daylight bulbs. Hang it 5-6 inches above the tree and keep it on 16 hours a day. I say ideal because it's really the cheapest of useful one (don't use fishtank lights!), and because if you're anything like the rest of us you'll end up with a lot more trees anyhow. Other than that, a high wattage spiral fluor. (do they make them?) would be the next best thing (in a table lamp). Are you humidifying properly (without spraying)?