what is the platinum edition

is it for proph and eotn, or else?

it contains Prophecies and Eye of the North only.

It gives you both Prophecies and The EotN expansion.

if i already have eotf but no proph
how can i buy it (proph)

i mean from game store

Sign over your immortal soul?

Seriously though, it's pretty simple. Find a shop that sells it (GAME or Game Station), make sure it's a sealed one, buy it, use the key on your existing account.

no shops (in my country) sell GW

Well then, you may have to buy it from the internet (online store, amazon etc.).