university laboratories — They process a lot of mice as part of their research programs, and tend to publish high-quality information too. But they also can afford more expensive equipment (e.g. CO2 pressure regulators, industrial tanks, etc) that I can't necessarily afford.
snake-keepers — Some snake-keepers breed their own feeder-mice. They tend to use cheaper equipment that I might have access to.
personal experience from working at a wildlife rehab facility — I got to observe their euthanasia setup in action several times. They're an organization that's deeply committed to humane care, and their setup seemed to mirror laboratory setups.
personal research about suicide methods — I know this is unfortunate, but as someone who deals with severe depression periodically, I have spent a lot of time researching suicide methods.
It seems like carbon dioxide asphyxiation is a solution that a lot of people recommend.
Other articles:
a YouTube video by a snake keeper which demonstrates using dry ice to achieve carbon dioxide asphyxiation