I have used Dr Earth brand and CitrusTone brand fertilizers available at my local nurseries before. Don't know if they are the best or not. They are organic. And not cheap. But, I grow less than a dozen trees so I do not use too much of it.
I am not sure how many trees you are growing - because, if it is a lot, there is an organic growing forum on gardenweb, where some one would be able to tell you how to formulate organic citrus fertilizer cheaply by combining ingredients easily available.