Bread left out in the open air will collect natural yeasts. Not as good as brewer's yeast strains or even baking yeast, but it will work.

It is also worth mentioning that these guys have an extremely low tolerance to alcohol after being locked up for so long, so they don't need much or a high percentage to feel it. Also, the percentage of mental illness and/or mental health meds being taken and also abused impacts the outcome and severity of getting hooch drunk.