Unless you hack or transfer them ,via Pokebank , there is no legitimate way to get Palkia and Dialga.
Dialga being distributed to owners of Pokémon Black and White and
Pokémon Black Version 2 and White Version 2 between August 19th and
September 8th, with Palkia following on September 9th until September
29th. The event will then finish with Giratina's distribution, which
will be from 30th September until 20th October.
However there was an event for BW and BW2 in which you could get Shiny Palkia, Dialga and Giratina.