In platinum is there a stone to level up an chimchar?

There is no stone but you can use a Rare Candy. That will make it go up 1 level, or you can get a Super Rare Candy. 1 in 100 games of Diamond, Pearl and Platinum have it replaced instead of the normal Rare Candy. Super Rare Candies make your Pokemon level 100 straight away, no matter what level they were on before hand.