With normal fluorescent tubes, you aren't going to find higher wattage lighting than what you have now. Even if you found a bulb that was rated for more power, the current ballast in your canopy simply wouldn't drive it any more than it is doing now.
If you're wanting more light for strong plant growth, look at a new canopy. Here is one that would fit on your tank and provide 2.4 watts per gallon.
35" Plastic Lighthouse Compact Aquarium Light, 2-36 watt bulbs - $80