Where can I find ephedra?

I'm curious, where can I locate ephedra? I've heard it's a popular supplement for weight loss and athletic performance enhancement, but I'm not sure where to start my search. Do health food stores usually carry it? Or should I be looking for online retailers? Is there a specific type of ephedra that's considered more effective than others? And finally, are there any precautions I should take when using it? I'd appreciate any insight you can provide.