Forget going to places like Costco and Sears, or even Tirerack. Go to someplace like AutoCustoms (at the old Eurosport) or even Revolution Motorsport.
Trust me, they are one of the better choices for those looking to find those aftermarket tires that are more affordable, yet pretty darn good.
I am willing to bet they will find the tires you need for your 20" wheels for your Tundra. Autocustoms did for mine, at nearly half the price that Tirerack quoted.
And they got it in one day. Autocustoms is behind the Hotdog place across the street from the Volvo dealership.