Groceries are a challenge in the north.
Dawson Creek, Ft St John and Fort Nelson have good stores.
Watson Lake has one grocery store (Super A), which is a decent store but not for organic or fresh/fresh stuff. It is also expensive. I usually stocked up here though as we were heading south. Northward I would wait for Whitehorse.
Nothing decent in Teslin.
Whitehorse has numerous grocery stores and organic and fresh foods are more common. Fortunately, the selections for organic and gluten-free are improving at Extra Foods, this being the best store for organic produce.
The Great Canadian Superstore has a great deal of food and drygoods, but seems to have a problem keeping their store free of varmints. Their selection of organic is limited.
The Super A food outlets are also good, but not too much organic selection. Best meats at Super A though. The meat at Extra Foods and the Superstore have an odd smell when being cooked and also an odd taste, for me anyway.
Walmart has a decent selection of canned goods, dry goods, diary and breads. Most reasonably priced store in Whitehorse too.
There is also a great farmer's market in Whitehorse on Thursday afternoon/evenings. Very good stuff there.
So, when I go into "town" to shop, it is Walmart first for the basics. Then on to Extra Foods for the produce and items not available at Walmart. Then on to Super A for meats.
After leaving Whitehorse, there is nothing along the Alaska Highway until Tok AK. Nothing decent along the Klondike Highway to Dawson City either. Some of the gas stations have limited canned goods, frozen breads, etc.
In Dawson City there is one small grocery store, but don't count on getting good produce there. Stuff needs to be trucked all the way to get there.
The grocery store in Tok is bigger but still limited. I feel it is dirty as well, so don't enjoy stopping there for food stuff.
There is a wonderful Overwaitea in Delta Junction. Great produce. Good deli in the store too.
Then Fairbanks or Anchorage are your next good stops. Fred Meyers, Safeway, etc. I noticed in Anchorage at the Fred Meyer in March 2014 that they have improved their organic selection.
Best option is to take as much non-perishables with you that you can. I used to pack every nook & cranny with food when we headed north simply because the selections aren't that good here.
Fresh produce is trucked up the highway and then stored until put out to buy, so "fresh" isn't always fresh-fresh. For dairy products and other dated items, check expiry dates closely. It isn't uncommon to see expired meats and products out for sale.
Hope that helps.