You can buy melatonin in a variety of doses over the counter. Its normally near the vitamins and supplements part of the store.
Note higher dosing is NOT better...most OTC melatonin supplements give too much. 0.5 to 1.0 mg should be used 1-3 hours before bedtime.
http://www.talkaboutsleep.com/how-to-use-melatonin-correctly...
Number of hours are all over the map. Hey, if you feel its effects within 30 minutes, then that's what you should use...