Just a reminder that a quarantine tank is always good to have, and an absolute necessity if you are buying from a source you don't entirely trust. There is no purchase that's a bargain if it causes you to loose a tank of healthy fish.
^ This is the best post/advice on this thread so far. A quarantine tank removes a lot of the doubt you are be having about getting fish from a source you don't entirely trust.
I've purchased fish from all 3 mentioned big box chains (plus 2 local mom/pop LFS). Sometimes they've worked out great, other times not so great but you have to keep in mind that Petsmart, Petco, & Walmart are HUGE chains & their standards regarding live stock will vary somewhat from individual store to store.
I have 5 Petsmarts within a 20 mile radius of me, 3 of which I have frequented & their is a noticeable difference in live stock care from store to store. Same thing with Petco, I've got 2 that I frequent in my area & although they are both Petco & Petco HQ holds them to the same standards, there are noticeable differences in their livestock. Walmart I'm a bit more leery of. Only ever purchase fish from one of their locations (not all Walmarts sell live fish) & they worked out fine but being that Walmart is not a pet specialty store it only leaves to reason that they'd have the least experience regarding live stock.
Also, in addition to location, timing plays into it as well. I've been to Petsmarts & Petcos that had great looking tanks/livestock one month only to return a month later & see tanks with questionable water quality or infested with noticeable diseases
i.e. Ich, fungus, fin rot. I'm sure things like changes in staffing, scheduled maintenance, or local suppliers all play into it.
With all that said, if you have a quarantine tank set up & the price is right, their is no harm in picking up a fish from anyone of these stores as long as you quarantine your new purchases for a couple of weeks (2 - 3 weeks) & observe them closely before adding them to any of your other tanks.