Try this one:
http://taylorandfrancis.metapress.com/app/home/contribution.asp?wasp=b8790bd1646e412cbe02e87f6564d0a8&referrer=parent&backto=issue,6,8;journal,12,22;linkingpublicationresults,1:105324,1For macro-economics/strong middle class correlation:
This is an academic journal paper that costs $$ but you possibly
can get it through a university library
http://www.springerlink.com/app/home/contribution.asp?wasp=79eaa04648cf4453958fb1f439c57ad3&referrer=parent&backto=issue,3,4;journal,15,34;linkingpublicationresults,1:102931,1http://www.vcharite.univ-mrs.fr/GREQAM/pdf/working_papers/1999/99b11.pdfhttp://www.nuff.ox.ac.uk/Economics/Growth/refs/inequal.htmThese are basically a correlation to a strong middle class and a strong GDP
I think Vermont should be a good case study, but I believe the above applies also to a local economy because it is a "macro" economy
by itself when buying locally.