There is no way to obtain Munchlax in the Wild on the Gen V games. You will have to Breed a Snorlax holding a Full Insense to obtain one.
During the season of Summer, you can trade a Cincinno* with a person in one of the houses in Undella Bay with a Munchlax holding Leftovers.
Those are the only two ways. Credit to 4S for the quote.