It seems really hard to get the specific AV Pokemon candy, especially the legendaries, so how do we get Pokemon-exclusive AV candies?
These candies that are species specific are found by combo catching a Pokémon. The Candies are dropped sparsely and they correspond with the Evolution line being catched. Additionally, for every 50 Pokémon of a Species you release, you get 1 candy of that Evolution line. The Final way to obtain a Species exclusive candy is by walking a Pokémon you want a Species candy for with in the Poké Ball Plus.
Hope this helps!