There isn't a cemetery named Monrovia, but Indian Creek Cemetery and Mount Zion Cemetery are probably the largest cemeteries in that area. Indian Creek is on the corner of Indian Creek Rd. and Booker Rd. (the part of Indian Creek Rd between Hwy 72 & Old Monrovia Rd has been renamed Providence Main). Mount Zion cemetery is on Mount Zion Rd. behind the church of the same name. If you take Jeff Rd north from 72, turn left at the intersection of Blake Bottom Rd./Mount Zion Rd (it's Blake going east, Mt. Zion going west) & Jeff.
There are several smaller cemeteries scattered all around the Monrovia/Jeff area, is there anyone in particular you're looking for? Have you checked Findagrave.com to see if they're listed?