Welcome to Sailnet. You may wish to post this question in the Lake Huron forum under "Destinations".
I sailed from Midland to Bayfield Ont 2 years ago stopping in Thornbury, MacGregor Harbour, Lions Head, Wingfield Basin, Cove Island and then a straight overnight run to Bayfield.
So what?, that wasn't your question.
I know there is a well marked inside passage through the 30,000 islands up the east side of Georgian Bay with many stops along the way for provisioning and anchoring. I would strongly recommend buying the Ports guidebook for Lake Huron which includes Georgian Bay and the North Channel. It has excellent descriptions of what you are looking for.
Once in the eastern end of the North Channel, the main provisioning stop is Little Current. It has everything you would need. Killarney is a good stop for fuel, pumpout, ice, laundry, beer etc but groceries are marginal. Gore Bay is apparently good for provisioning but I haven't been there.
Start a new thread in the Lake Huron Destinations forum and you'll get lots of enthusiastic advice. I know several of the people who have posted there regularly are from the Midland Penetang area.
See you up there in July!
If you're not in a rush, then take your time and enjoy the cruise. A nice route to take is the Small Craft Passage. You can catch it right after Point au Baril at Hang Dog Channel (I believe that's what it's called) you can then anchor in the Bayfield inlet for the night. The next day follow the buoys out past Byng Inlet, behind The Chickens and into Collins Inlet. Take that passage if you can, it's very scenic. You'll exit near Killarney. Killarney is very expensive to provision, so don't if you don't have to. Just past Killarney is Covered Portage, it's a great place to anchor and it won't be busy at this time of the year. Go through the Landsdowne Channel to get to Little Current. I recommend you hang a right prior to going to Little Current and visit Baie Fine. Once in Little Current, you can provision, get fuel at Wally's and grab a bite at the Anchor Inn. Many great anchorages on your way to Gore Bay and beyond.
Like CS said, buy the Ports book. I'd also recommend you get North Channel charts as there are many rocks, shoals and reefs.