We see that, too, at those types of stores. Some people
use Miracle-Gro potting mix for potting up seedlings to
4" pots or similar. YMMV.
I used some Supersoil one time. It collapsed as soon as it
got wet. I would pot up a seedling, set it in a little tub of water
to soak up water from the bottom, and the next time I looked
at it, the little pot appeared to be only half full of potting mix
and the top of the seedling was no longer above the top edge
of the pot.
Relatively close to you (IIRC), this place lists high-quality
FoxFarms products:
http://roomtogrowav.com/
(Typical online prices for 1 cubic foot of FoxFarms Light Warrior:
http://www.google.com/products/catal...d=0CDgQ8wIwAg#
)