What kind of antibiotics do you need - do you know?
If you do know then find out what disease humans get that needs treated with that antibiotic, check out the symptoms and go to your doctor.
While they are not supposed to prescribe antibiotics without determining the actual bacteria causing 'your' symptoms, they probably will prescribe you something broad spectrum, especially if you push them.
Urinary tract infections and respiritory infections should normally get you a week or ten days (human) supply of broad spectrum antibiotics.
I'll leave the morality/legality of the issue to you as you know your country better than I do.