You can easily buy frogs from the pet stores. They will not just get rid of your slugs and snail problem but they eat mosquitoes and flies too. Wild frogs are an even better option because of the great hunting abilities that they possess.
Frogs need ponds to breed. Introduce frog friendly features in your garden, such as a pond, log piles and compost heaps. Do not add fish to your pond as they feed on the spawn. If you do not have space for a pond, a shallow basin of water will also work. Remember to keep the basin filled and the water fresh.
Make conditions favorable:
Frogs breathe and drink through their skin. Chemical pesticides and insecticides are all poisonous for frogs and toads. Absence of such chemicals in the garden will make it a safer environment for the frogs and toads.