Field mice are found throughout Davis, house mice share our homes, lab mice are on campus, and there are a few friends of the Wiki like Little Girl. Some parts of Davis also have rats and new world mice (aka peromyscus), neither of which are actually mice (mus musculus), as well as a variety of other rodents or small mammals which are not mice but sometimes lumped into them. A wide variety of rodents, including mice, can be attracted to dried food and compost piles.
If you see a wild murine being, please leave it alone, or — if you absolutely need to — transport it as delicately as possible. They are inoffensive creatures but they can carry disease, especially peromyscus, which harbor hantavirus. Wild mice have been known to suffer complete organ failure or cardiac arrest due to the stress of being handled and/or caged; they are by nature very timid creatures and hand training them requires very consistent effort from a young age.
Domestic mice cannot be released into the wild. While it may be a well intentioned act, you are making them suffer a horrible death. Please try to find a home for them if you can no longer care for them, and be responsible enough to provide them with basic medical care due any companion animal.
Veterinarians Who Treat Mice
Obtaining Pet Mice
Private breeders can be found in Sacramento, but there are no RMCA or AFRMA member breeders who sell or trade mice in Northern California. Properly bred mice (called "fancy mice" to distinguish them from feeder mice with dubious genetics) are multigeneration domesticated mice with a good genetic stock. PetCo sells both fancy mice and feeder mice, although they do not always have fancy mice in stock. Feeder mice are often products of incest and have neurological issues, display non-mouse like behavior, and will suffer a number of health issues over their lifetime. If you are not buying a mouse from a breeder, inspect the mouse carefully for trauma or health issues. PetCo will send a sick or impaired fancy mouse to their vets, and will find a home for a fancy mouse considered unsellable.
Note that the Webster Mousery in the Bay Area moved to Illinois, but is still listed in many breeder lists as being in California.