As much as I love Costco and they are my default "go to" store, for tires it's Discount Tire.
That practice same as my son. He has a Costco membership, but for tires in the ATL area, it's always DT.
For me no Costco nearby, but even if there was and I a member it wouldn't change where I buy tires, DT. They've given me no reason change, and over the years many reasons to stay.
And as an aside, reading this subforum for many years, even some of those that tout Costco as the greatest thing since sliced bread (my words) for tires, I've seen pics they posted having purchased and installed tires at DT/AT. When queried about source the pics of the inside shop area, no answer. Perhaps ashamed or something similar to admit went DT/AT for tires.