you can get penicillin for horses, I know, without it being prescribed, but . at a feed store? No... I never saw it there.
And... while I am thinking about it - since I am allergic to about 12 antibiotics: shouldn't Jack have asked D if he was allergic to any specific antibiotics before he stuck him full of what he had in his bag? If he had done that to me it would be the last chapter of the story. And a LOT of people are allergic to the penicillin/ampicillin type.