grade antibiotics through pet and fish stores. They are the very same drugs that end up in your pharmacy. They have a shelf life of several months to a couple years.
Many preppers have figured out how to make penicillin tinctures, teas, etc. that work well against many non-resistant bacterial strains. A few prepper communities have even become sophisticated enough to synthesize their own penicillin, amoxicillin and tetracycline. Most preppers also know how to make and use colloidal silver for topical anti-bacterial purposes.
Prepping isn't just about beans and bullets.