It depends, if you're looking to buy in bulk then http://www.essentialoils.org/
is your best bet. For smaller quantities, www.camdengrey.com
has great prices but they charge outrageous shipping. I just paid $23 for 2.2lbs of tea tree through the first link but I make soap so I buy in bulk. It also depends upon where you live. You can buy locally at health food stores but they are so expensive, sometimes it's cheaper to pay for shipping. If you let me know where you live, I can let you know suppliers in your area.