i got mine at the "nearest" hydro store, but dude told me that they used to sell it everywhere on the east coast like 15 years ago, then there was an issue with the EPA due to its organic compound. so for a while you could only get it in cali...
ask around your local hydro shops, if they are knowledgeable, they will have it or know where to get it.

stinkyattic recommends a mix from agway, and we have an agway down the way from me so i had a buddy use the soil she recommended. it seems to be workin rather well also.....