Hello,
I am located in Rochester Michigan and I can tell you that it is quite difficult to find a good wholesaler who is willing to write commercial lines for contractors in Michigan. I basically have to sub out all of my commercial work to another agency and take a huge commission cut just to write it.
There are several wholesalers with national programs for small and medium sized contractors.
I work with SIS and I believe they cover 48 states including Michigan. Appalachian Underwriters, Inc does as well. Nationbuilders writes in MI. There are several.