That's an option. But I'd like to know how much duty would cost me too see if it's worth the risk. I don't want to get flagged at the border. Then get hassled every time I cross.
If you're going to buy the tires in the U.S. then having them mounted down there is the best bet as long as you don't have to bring back your old tires. I've done this several times with no issues at the border, They would have a hard time proving you bought them there to begin with. Who's to say you didn't buy new tires before you entered the U.S.. It's not what you know, it's what you can prove.
In answer to your original question, You would not be charged any duty as long as the tires were made in the U.S., but the catch is vey few tires are made in the U.S., I think the only ones are Goodyear and some Cooper models.
Here's a link to a tire company in Canada that has good pricing and real cheap if not free shipping.
https://pmctire.com/en