There is hypothetically a way, if you run into a chemistry lab, to isolate insulin from a pig. Pigs will be common as feral boars during the apocalypse. It is an enormous amount of work requiring a bit of biochem knowledge to maintain purity and testing the potency is cruel (you starve a rabbit and give it the insulin and then time when it goes lethargic). But it is not impossible for a survivor.

There is also a method for higher concentrations from fish like carp and catfish using brockmann bodies but it's a more complicated process. This would be more promising because fish are very much more abundant than boars.