Medical leeches are expensive, and they are a different species than bait leeches.
Medical leeches are active hunters who look to blood.
The leeches gathered for bait are scavengers that eat insect larvae and anything dead that falls to the bottom of the pond.

Bait leeches are caught by placing kidneys in a folded sheet metal envelope that is dropped into the pond in the evening, and picked up the next morning. They are sorted by size, large ones are sold, small ones are sometimes fattened up in man made ponds where they are fed scrap meat. It is a seasonal business and even farm fattened leeches are born and caught in the wild.

T.Butlerii love leeches. No, they LOVE leeches, but I haven't found a satisfactorily clean source- wild leeches cary quite a few microscopic parasites and diseases.