First of all, Phentermine is a prescription drug that must be perscribed by your doctor. It is only issued to obese patients with a BMI over 30 (I believe). Essentially for people with more than 25lbs to lose.
Even then, most doctors are not willing to prescribe it haphazardly because there are side effects that come along with it. If you are on it, you should be under doctors supervision.
Now, you can search for phentermine online and you will get alot of results - but look closely at the spellings and you will realize that what you see are "herbal weight loss suppliments" that are not regulated by the FDA. Essentially these are not much different than your over the counter weight loss pills like trim spa, xenedrine and all the others. They vary in the ingredients they use and many use what they consider proprietary ingredients which you really do not know what they contain. Its a crap shoot whether they really work or not.
There is alot of information on this site regarding diet pills that can give you much more detailed information.