Where is the "head shop"?

[Douglas Bigham]

I've probably just over looked it.  But, I can't find "head shop" anywhere (OED Online most importantly).  I wanted to know if the "head" part related to (Grateful) Dead-heads, but I'm at loss here.
I know this is a common word (a place where one buys marijuana-smoking accessories) and I know it is NOT exclusive to Southern Illinois. Am I missing something?  Do y'all know this term?  Can anyone help me on the history?

-dsb
Douglas S. Bigham
Southern Illinois University - Carbondale