Spilotes pullatus, maybe? Unfortunately, I think we've found most snakes big enough to eat rodents, and ones that breed year round still only breed once. Like Kenyan sandboas, they can breed at almost any time, but just once a year. And don't get any ideas, they don't get big enough to feed a king! Hmm... but maybe I'll buy a coral and feed it my surplus. No... I couldn't bear to feed something that wholesales at 35$.
I know that there is probably some kind of tropical rat snake that can be bred year round, with little expense (we just haven't found it yet).