As Melmarsh says. You need a prescription for antibiotics anyway, and your vets would want to be sure you understood how and which to use. Just buying something which may be suitable, just in case you need it, isn't really a responsible approach. Antibiotics are specific, and their improper use is what can lead to the major problem of bugs which are resistant to all AntiBs. So Pen and Strep would be useful in some cases, but in others Oxytetracycline would be appropriate, and in other cases no antibiotic would be needed. Until you know more you need to rely on your vet for advice and help.