Items that are useful for Ghost-type Pokémon:
Spell Tag – Just gives a little boost to Ghost-type attacks.
Spooky Plate – Just gives a little boost to Ghost-type attacks.
Reaper Cloth – Helps evolve Dusclops. Not very useful, since all internet functions for anything below Gen VI. Still, I believe face-to-face interaction still work, so you could try that.
How to obtain/evolve Frosslass in Platinum:
Step 1: Catch a Snorunt: The first thing you need to do to obtain Frosslass is to catch a female Snorunt. It must be female, otherwise it cannot evolve into Frosslass. You can catch it at any of these locations.
Step 2: Get a Dawn Stone: Next you'll need a Dawn Stone. The Dawn stone can be obtained in Pokémon Platinum on Route 212, Route 225, or Mt. Coronet.
Step 3: Evolve thy Snorunt: Finally, evolve your Snorunt into a Frosslass! Open your Bag, go to Items, and select the Dawn Stone. Select "Use", then select Snorunt. Do not press B whilst Snorunt is evolving, or else you will have to go look for another Dawn Stone. Good luck, adventurer!