I live near Traverse City and have bought briskets at Sam's, Wal-mart and a nearby IGA. The IGA is best because I think the quality is better. Sam's is ok too but they usually sell only the flats. Wally I'm not too keen about. Looked at a full packer a week ago and it was Excel brand and not graded that I could tell. Wally has a good price but the meat might be of lower quality.
I think like Smokin' says, you need to ask people or at least go to various food markets. I came across a new brand of ribs the other day (Meadowbrook farms of Illinois) that were really better than any I had ever bought. So I'm sticking with those.
The IGA I go to is really out in the sticks but are pretty accommodating. In the summer they carry most everything but in the winter they won't have brisket, just ribs and butts. They will order if you want but a case is about 4 briskets. If you can find some other BBQ buddies to go in with you then you have it made and can buy pretty much what you want and where you want based on putting in an order.
Just look around.... you'll find the right spot. And then keep looking for something better.