I remember seeing fish at Walmart, but that was a long time ago;and I think that none of the Walmart stores here sell live fish anymore.
They have all kinds of fish supplies, and maybe even the tanks for fish, but no actual fish. I think this is something that is trending away from non-pet stores, and probably that is because of the extreme loss of fish, and the lack of care for them.
It used to be that a lot of the stores sold goldfish, at least; even if they didn't have any other kind of tropical fish. Since the goldfish were always cheap (I remember buying them for about 29 cents apiece), it was not a big loss when the store had some of them die due to lack of care. Once they started carrying the more expensive tropical fish; then every dead or sick fish was a bigger financial loss, and I think that is why many of them are no longer selling fish.