Stupid this will sound. Wash them. Put a couple in a 10gallon terrarium. Give them places to breed. Keep the offspring. Make them grow. Release parents. Keep a couple of offsprings for breeding purposes. Feed the rest to your frog. Voila. Breeding slugs isnt hard. They pretty much do it on their own---literary. Keep temps at 72 degrees. Dont let it get dry in your terrarium. But dont let it get sopping wet where the friggin substrate is like mush. Becarefull about live plants. Some plants are poisenous(especially house plants) which when ingested by snail/slug it may get those attributes if not killed. Which then may poison your frog. Feed it thin slices of zuchini cucumber, slices of carrots, potato, tomato, apple, and kale. Personally Id chop them up in little tiny bits, and put it in a feeding dish.
Good luck.