I keep a multidose vial of Ultrapen, a long acting penicillin, at home. It will cover most of the common pig infections and because it's long acting you usually only need to give one injection: helpful when you're dealing with an unco-operative patient!
I get it from my vets. If you're not confident giving injections to stock I would recommend getting some training, formal or informal, from someone experienced. I'm a doctor, an anaesthetist in fact, so I inject people for a living, but most of my patients who weigh as much as my adult pigs can't move as fast!
You can buy appropriate sized syringes and needles from farm supply shops.