I'd go to Discount Tire any day.
I thought Big O would do trailers too, but then I found out the truth the hard way.
This is a good opportunity to learn the simple process of removing and re-installing a wheel yourself. All Airstream owners (especially a single-axle trailer owner) should know how and carry the tools needed, including a torque wrench.
You'll love the Michelins. After seven years of experimenting with various brands of "ST" tires (Goodyear, Carlisle, Green Ball, Tow Max, etc), I won't use anything but the Michelins.