You could probably call these people and ask if they carry
Pro-Mix or Sun Gro products:
http://www.idiggreenacres.com/soil.html
(Sun Gro products:
http://www.sungro.com/products.php)
You can drill down from the Sun Gro page, and they give
you a lot of information about the makeup and applications of
the different product lines.
L&L Nursery Supply in Stockton may carry this stuff, but I do not
know if they sell retail. (For some of these places that sell supplies
by the pallet load, asking for one bag of something is like
walking into a convenience store and asking the clerk if you
can buy one peanut.)