I've found a few decent sites that sell essential oils and they seem good quality..
Although for people attending Stoke on Trent college I personally think the prices and range of oils they have on Beauty Reception are very reasonable when you consider postage costs of online Distributors! You've probably heard Vicky mention Aromantics and Tisserand.
Feel free to add any more sites you think are good for Essential oils!
http://www.quinessence.com
http://www.tisserand.com
http://www.aromantic.co.uk