Where can I buy one?

You can buy one off http://www.floridadriftwood.com/store...they're pretty expensive, and you have to reconsider if you really need one after all?

I bought a 300W cable heater at M3 for $60. I connected it to a regular $5 light dimmer, which lets me control the heat output in a continuous fashion. Also, I have it on a timer, so it kicks in only at night, when the lights don't heat the water. I didn't use it much yet, but Winter is coming.

Well everything I've read seems to indicate that a substrate heater is a good thing to have. Do I need one to have a nice planted tank?

Absolutely not. There are probably more people who don't have them and have nice tanks than people who do have them and have nice tanks. If that makes any sense. :confused:

Also consider what plants you are growing. If you have any sword plants or crypts you are going to have problems if you ever have to move them. The roots will get tangled up in the cable in no time.

It's a great idea... gentle nutritious currents, warm feet, and very efficient heat distribution, but I don't think there is hard proof that it actually improves plant growth, much less that it is required to have a planted tank.
I will not use mine for about 8 months of the year because I don't need a heater during that time... If I had to start over, would I use it again? I don't think so, for the very reason that Rex stated.

I'd have to agree with Rex here too... Any sort of heavy root feeding plant will wrap itself around the cable, so it can get pretty messy in no time.

On the other hand, if you are using a heater a lot of the time, or have to keep a warm tank, like for discus, bottom heat is something plant nurseries will use to get a jump on growth in the spring.

Will this be for a tank in Oregon or in Alaska?