Correction to lianad's list:
Liberty-Flights always gives a 20% discount on your order. You can find the code on the Discount Code tab of their site. Just copy and paste. They also give you credits for reviewing what you've purchased.
You should also be aware that the Discount Codes list isn't kept up-to-date, so some of the codes may not work. The best thing to do is check the Registered Suppliers
threads so see if they have a current code.