Wal Mart has it. But it comes in small bags and is expensive. Go with any thing thats light and fluffy. Some people say not to ure miriacle grow potting soil but, I got very good product from it.
You can buy walmart soil, but don't get anything with "Miracle Grow"on it. If it's dense that do a 50:50 ratio of soil-perilite
Foxfarm's "Ocean Forest" is probably the best I've ever used, but it's a little "hot" for babies in small pots (dilute about 25-50% with "plain" potting mix untill they're about 10-12" tall). I've always wanted to try Kellog's "Patio Plus" but never got around to it! Both are full of organic nurients and can feed your plants for a whole grow with some additional flowering "tea" added in the later stages.