Go fight audinos north of mistralton city. They give a ton of experiance no matter what
level your pokemon is. They are found in the rustling tall grass, not regular grass, you
will be able to tell the difference.

This is pokemon platinum, not pokemon black & white.

To answer your question, minepose98, you can go to these areas for fast grinding:
Iron island
Routes 225, 226, 227, 229, 230
Stark mountain

They have high leveled pokemon, so you can grind easily there. Also, remember you have a
vs seeker, so if you are in need of fast grinding, then use the vs seeker for rematches
and massive exp.

My mistake, i apologize.

There is also an item that increases exp, i think it is the lucky egg. I don't think you
can get it in the game form what i remember. If you can get one, give it to the pokemon
you would like to train, to earn some extra experiance.

Can be obtained from a wild chansey and there's a 5% chance of getting it. You can easily
detect one if you have a pokemon with the ability frisk. Frisk checks any held item of the
pokemon once it is sent out in battle.

Should help with the item hunt.