ask any relatives if they have woods or anything like that that you could dig some dirt up at. Even if it means paying them. Dirt you get yourself will almost always be better than store bought.
If you must purchase dirt, your best bet is to go to Lowe's home Depot etc. And get a lot of top soil and play sand. Make sure there are no chemicals etc in them either. Organic topsoil. Get about 3 or so bags of topsoil for 1 bag of sand (assuming they're the same size bag).
Run/strain all the dirt and sand through a screen (egg crate works great for me) before using it. Mix the dirt very well too (most often the dirt you get from lowes/home Depot will have lots of clay.)