Costco. I have bought probably 20 pairs there over the years.
Ha! Look at 82 eyes over here! Nerd!
And yeah COSTCO is great but I've found Wal-Mart to be fine as well.
Though on my last trip to the Wally vision center the ophthalmologist was Kenyan and he spoke in this Englishy-African accented, fast, sing-songy voice which made him hard to understand. Like I would have to pay intense attention to him and then once he was done speaking play it back in my mind but slower to get his meaning.
He was great at his job though.
My last couple of pairs were bought at local places, which were great but don't do you any good as you don't live here.