Some pet shops sell frozen mice and rats for snake food. Or you can order them online as snake food. Small mammals like rats are perfect for that.
You might also be able to source things like whole rabbits from butchers. But I’m not sure if they would sell them whole. I couldn’t get any whole animals from my butchers because apparently dead animals are a “bio-hazard” unless unless they are in 12 separate pieces…
It’s the guts and brains that are an issue. Lots of nasty, nasty diseases that can be carried in there. Same thing that makes it hard for people to raw feed their dogs and cats properly, that unavailability of organs. Particularly the digestive tract.
Depending on laws in your area, roadkill may work. Some places you can’t legally pick up roadkill, some you can. If it’s fresh, freeze it for awhile to kill the worst of the pathogens, then it’ll work fine as a practice specimen. Just don’t eat any part of it.