You get it from honey trees
Munchlax > lvl up with max happiness > Snorlax
level it up
munchlax does not evolve at a sirten lvl it evolves when its friendship is fully maxed out
Your rival in diamond/pearl/platinum
a honey tree
in honey trees
trade it.
There is no trainer in platinum that is willing to trade a munchlax.
It evolves via happiness
You can slather some honey on a tree and it might come. (Munchlax is very rare though)
munchlax is a pretty hungry Pokemon so i guess a lot of honey
To find munchlax put honey on a tree and wait until a munchlax appears or breed snorlax.