You can' get Infernape, it's an exclusive starter you can choose at the beginning of the game.
You cannot find an infernape in Pokemon emerald for that Pokemon you need Pokemon platinum, pearl or diamond.
Infernape does not exist in Pokemon LeafGreen. Infernape didn't exist until Diamond/Pearl/Platinum, which was released after LeafGreen.
Copy your team with infernape in it from Diamond Pearl or Platinum
Chimchar Monferno Infernape Ponyta Rapidash These are the the only Fire-type Pokemon found in Sinnoh
if you choose the grass starter then your rival will have an infernape or when you go to the Pokemon league , the 3rd elite four is a fire type user . he will have an infernape.
Rival
blast burn
Infernape Venusaur Blastoise Palkia and 2 others
Either moltres or infernape
darkrai shaymen infernape and staraptor
ither you have to start with chimpchar or trade
You must breed a Monferno or Infernape with a Pokemon to get the chimchar egg.