Tires in the US are cheaper than in Canada so the first place I'd ask if the dealership where your bike is currently getting fixed. Northern prices are generally higher for all bike related stuff so I wouldn't imagine that the prices will be great, but I doubt you will find much better.
Cheapest option is to buy online and have them sent up to the US, but you need to be in the state for a while to wait for shipping time. Shipping into Canada is a bit costlier and time consuming, but still is cheaper than buying from a dealer.
Here's one of the places you can try in the US.
Welcome to Southwest Moto Tires
Other option if you want to burn some more tread is Whitehorse, couple of bike shops in town that change a lot of tires and are very friendly. Can't remember what a rear Tourance in Canada costs, but it is significantly more.
Try Yukon Yamaha to see what they are asking for a tire.
120 Industrial Road
Whitehorse, YT Y1A 2T9
Good riding, if you are heading down south and are looking for a place to stay, drop me a line.