Because you posted an email, you may have a source already, but if you don't, have you considered asking a farmer? There must be parts of Michigan that are rockier than others. If you see an accessable rock pile, talk to the farmer about taking some rock. He or she might think you are crazy, but we're all a tad crazy anyway, so that shouldn't stop you :-).

I get my rock from a farmer. The only way I even knew about their enormous rock pile was by talking about needing rocks to the farmer's son. Start talking to people you work with, neighbors, etc. They might know someone who will let you on their land.