Pro mix organic soil is about $8 a bag and is the best I've found but I haven't tried the high dollar fox farms stuff($20 - $25 a bag). I was more than happy with pro mix.
Regular pro mix is $1 or $2 cheaper, I did get better results with the organic.
Here is there website you can see if anyone close sells it, mostly your better greenhouses carry it.
http://www.premierhort.com/eProMix/index.htm
Some people used to have good results with something from walmart but you had to get a certain color bag and I forget all the details, you might read up and see ifyou can find anything.
The pro mix gets my vote.