Antibiotics can only be obtained from a licensed veterinarian. And in order for a vet to prescribe antibiotics, there must be an existing doctor/patient relationship...ie. the vet has to examine your snake or has examined your snake in the past. It is against the law for a vet to prescribe medication without ever having seen your snake (in other words, an exam is not simply "money in the vet's pocket," there are legal reasons).

p.s. I suspect the main reason your snake was on the prowl after a few months in the tub was because of his proximity to your gfs snakes (females?) and the arrrival of breeding season!