The Fish Mox stuff is identical to what they dispense at pharmacies. I have seen two different manufacturers' capsules in the Fish Mox bottles, and in both cases they are the exact same pill markings as antibiotics I have been prescribed on previous occasions. I have yet to see them in a local pet store, I buy it online. You might have luck with an independent pet shop though.
Yes, getting antibiotics as the result of a doctor's examination and prescription is preferable and responsible. What we are discussing here is a potential avenue to obtain what might one day be in very short supply, and by that I mean obtaining them before the fact.
There's a few different types available, each targeting different types of bacteria (Gram Positive, Gram Negative, etc). It behooves one to do a little research into what is effective for what. I used Fish Mox as an example because it is amoxicillin, a very common antibiotic frequently used as a first-line treatment for things like strep throat, abscess teeth, etc. Without modern medicine, something as mundane as a toothache could become literally life threatening if the infection manages to get into the bloodstream and become systemic. I for one prefer to have options in the face of an increasing likelihood of a dramatic decrease in the quality and availability of health care in the USA.