You need a certain wavelength from an LED light bulb. I've heard of certain poeple using 1 christmas light, but I don't think that would have the right wavelength.
There was a thread about it a little while ago in the DIY section (I believe the name of the thread was DIY moonlight.) There's some good info there. Besides, I don't think they're that expensive to buy.
They're fun to have though!