There really isnt much youc an buy prefab unless you want to buy the entire vapochill water chiller kit or such
you will need to gut a freezer or dehumidifier or a fridge and use the cold coils dipped in your resivour or another receptacle having the water pass over it making it effectively cool... you will also have to worry b out condensation if its a strong enough chiller
I've been thinking the innards of a refrigerated water fountain would make a good water chiller. You might be able get one cheap when a local school or other public building gets remodeled. You can also find them on eBay, but they're expensive.