The leaf stone is what you use. You can get one in Castelia city.
Pansage
It won't evolve until you've used a Leaf Stone on it.
Pansage is capable of evolving at any level because it evolves by using a Leaf Stone on it.
give it a leaf stone
pansage = leaf lampent = dusk
Each monkey will evolve when it has it's respective stone used on it. Pansage will evolve into Simisage with a Leaf Stone, Panpour wil evolve into Panpour with a Water Stone, and Pansear will evolve into Simisear with a Fire Stone. As with most stone evolutions, they don't learn any attacks after you evolve them, so I would reccommend having them learn the attacks you want first. :3
pansear, pansage and panpour
nuzleaf, eggsegute, gloom, weepinbell, and pansage
Gloom, Pansage, Nuzleaf, Weepingbel, and Exeggcute
Gloom, Nuzleaf, Exeggcute, Eevee, Pansage, and Weepinbell.
The only Unova Pokemon that evolves with a Leaf Stone is Pansage.
Generation 1: Gloom, Weepingbell, Exeggcute Generation 3: Nuzleaf Generation 5: Pansage