How do I find one that's likely to actually kill and eat mice, rather than kind of chasing them a bit maybe if it feels like it?
Farm cats are almost always good hunters. Get yourself a kitten from a farm, or from a place where there are resident mousers, like feed stores, bakeries, flour mills, etc. Make sure the kitten is between six and eight weeks old, so that it's had the chance to learn how to hunt from its mother, and, if possible, been socialized to humans.
The best hunting cat I ever had started life as a kitten from a feed store mouser. Never had to worry about critters getting into the chicken feed. Not only could she hunt down rats half her size, but was also the least fussy eater of any cat I've known.