I just bought a special plant growth bulb for the dentist tank, thinking it was the right wattage (I don't know what I was looking at when I bought it) but it's only 15 watts for a 30 gallon tank.* This is not a low light plant tank.* There is one java fern, but everything else needs like at least 2 watts per gallon I understand.
Has anyone ever seen an 18", at least 60 watt, bulb?!* It's a tall tank, so the light fixture uses a 18" bulb. GRRRRRR...
In linear flourscent bulbs, length is directly related to wattage. So the short answer to your question is no, you will not find the same length bulb in a higher wattage.
You are pretty much stuck with making something, or if you are not a DIY type person, getting a new fixture. Aqualight has some nice PC fixtures that should work nice.
Emma, I was looking for a bulb much higher in wattage than my current one (20W) over my 30G tank. I couldn't find one. There are none like this. Or if there are, they need a special hood/fixture. A HO (high output) or VHO (very high output) bulbs will not work on standard lighting strips. That means you either stick with low wattage or get a new lighting strip.
Wow offf ice you really know your bulbs... .. How about dualies? I see some tanks with 2 flourescents. You will probably get stuck buying one because I think trying to rig it together will prove to be too difficult unless you would go with indcadescent. I wouldn't suggest this because they put out a lot of heat and I don't think it is as "white" of a light that makes aquariums look good. I personally love my flourescents and so do my fish?? 8)
If you are willing to spend some money, buy one of those hinged glass tops for your tank and get one of these:* http://www.aquariumguys.com/30orbit.html.* I have one for my 37 gallon and it is awesome, plus the legs slide on a track so that they will fit your tank.* There are several sizes to match up close to the length of your tank.
My lfs has some 55w compact flurescent lights and double tube flurescent lights made by All-Glass Aquarium that might be 18" long maybe check your lfs for them.
Now what?!
.. How about dualies? I see some tanks with 2 flourescents. You will probably get stuck buying one because I think trying to rig it together will prove to be too difficult unless you would go with indcadescent. I wouldn't suggest this because they put out a lot of heat and I don't think it is as "white" of a light that makes aquariums look good. I personally love my flourescents and so do my fish?? 
8) 