I have found that all Walmart, Meijer, Petsmart, Petco, and similar chain stores are not created equal in regards to their fish. There are two Walmarts in my city and one is far better than the other due to a few employees also being fish keepers. One in particular has a young lady who I have personally witnessed discouraging people from buying inapproiate species. I have seen a few employees at a local Petsmart do the same, but I have meet others that were not so well informed.
Even if they do have some qualified people the fish don't seem to last long stacked up in all those small tanks. I reccomend hanging out a bit to see how a store operates its fish department. If it seems okay find out when they get new fish in and check the day after. If they have something you want get it out of there before it begins to degrade.
You might also find that some of these retail stores have under priced fish sometimes misidentified and you can get them cheaper. I saw several synodontis cats that cost $15+ at a pet store for $7 at a Meijer. I recently scored on some nice Golden Tinfoil Barbs that had just come into Walmart at about 6", for only $2.44. That is like a quarter of what they would sell for around here at pet store or LFS. A great deal that I will eventually trade in at a LFS for more than I paid.
Naldo