nowhere. you need to trade it from a different game
A Muk Isn't A Very Special Pokemon. It Evolves From A Grimer, Which In The First Who Pokemon Games(Kanto And Johto Regions) Aren't That Hard To Find. ^.^ But, It's Hard To Find A Muk And Catch It.
By evolving Grimer
Muk is the better pokemon
Which game? Grimer > lvl 38 > Muk In Firered and Leafgreen you can find them in Celadon City by fishing, but that is pretty rare, and the Pokemon Mansion eveywhere.
You find Grimer in the Fiery Path. They are rare is it may seem that they are not there but they are. Its worth the time getting Grimer then evolving it to Muk, because Muk is probably one of the stronger Poison type Pokemon that are in Emerald!
Muk is #89 in the national pokedex, and it is a Poison type Pokemon.
soul silver version
you can't catch muk in emerald but you can catch grimer in the fiery path and get him to level 30 and he will evolve in to muk okay
grimer evoles at level 32 to muk because i have 10
Muks are rare to find in the wild, but if you are lucky you will be able to catch one in the Pokemon Mansion in Cinnabar Island. You can also catch a Grimmer instead and raise it to level 38, then it will evolve to Muk.
The Pokemon Mansion is at the top of Cinnabar Island near the Gym. There you can catch any fire types like Vulpix. You can also find Muk and Grimer.
You can use some help with the Pokemon powers. Or capture the Muk forst because it's stronger. Watch out for the Mud trial!