Have you been to North Akkala yet? If I remember correctly at the East Stable there's a gaurd who gives you a side quest to go find him. Kilton is found in Skull Lake. After you find Kilton he will spawn at random areas at night (he tells you which areas he can spawn at), and you can exchange monster parts for Mon. You need 4,000mon to get the costume, most monster parts sell for 3-5 mon each.