|Why can't I buy a soil mix at any of the local nurseries or home improvement stores that provides good drainage for my plants?|
I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
The best way to provide good drainage for planting beds is to mix several inches of compost into the soil and repeat this periodically. There are a number of good potting soils for use in containers which do drain well. If you feel like your soil mixes are not draining adequately I suggest purchasing perlite which is the small white round substance used in potting mixes and adding about 20% more of it to the mix you are currently using. This should provide more than adequate drainage.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!