The company responsible for handling the Pokemon franchise, Pokemon Company, has just launched the official Pokedex iOS app in North America and Europe. The app will be in English, compared to the Japanese version that was released last month. Bear in mind that this may be the only portable version of a Pokedex you can get right now, as the (much cheaper) 3DS one has been taken off the 3DS shop.
The base version of the Pokedex iOS app is available on the iTunes store for $1.99, with the four additional expansions priced at $5.99 each. The base app only includes the Unova Pokédex (Pokémon numbers 494-647) while the four additional expansions include the Kanto Pokédex (Pokémon numbers 001-151), the Johto Pokédex (Pokémon numbers 152-251), the Hoenn Pokédex (Pokémon numbers 252-386), and the Sinnoh Pokédex (Pokémon numbers 383-493). This means a complete pokedex will cost almost $30.