It's very rare but in fiery path. Hope this helps :D
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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!
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
For Grimer to evolve, get him to level 38.
Grimer will evolve to a Muk at Level 38.
Grimer evolves into Muk at level 38.