I get soil at the farmers supply. Agway, specifically. I've checked a lot of other garden centers and the array of dirts is baffling; I've had good luck with Agway manure with humus + 30% perlite, Coast of Maine seaweed compost with perlite of course, farfard seedling starting mix, and a couple others.
When in doubt, add perlite. I say this because even if your soil sucks, has too much fert, or has an unstable pH, you can then safely flush your medium on a regular basis.