Research them online. Verilux sells several different types, and you can always check on Amazon. You can also find more detailed information on PsychEducation.com, which is a website chock full of information on light and dark therapy. What you want is a light box with 10,000 lux (which is a measurement of light) that features blue and white, or full-spectrum light. You'll also want to situate it where the light shines down on you from above and hits the bottom of your retinas, just like natural sunlight.

How long you sit under it and other details are up to your individual needs, and I'd call your pdoc once you've purchased one for further advice on using it. Be aware that the lights can induce mania in some people; however, it's worth a try because lack of light is a major reason for the depressions so many of us go through at this time of the year.

There is also the dawn simulator, which mimics the gradual onset of early morning light and retrains your brain to think it's July instead of January. This is the device both my pdoc and I use, and even though I'm still having some depression it is nowhere near as bad as it was in previous years. I get out of bed MUCH easier and in a better mood than when I had to get up in total darkness. I probably should use a light box too, but I'm going to see what the dawn simulator does in the late winter when my mood tends to really go in the dumper.

Good luck....hope this helps!