Yeah. There are a bunch of new pokémon that are evos of previous ones that are *not* in the Sinnoh dex of 150, and aren't available in-game before obtaining the national dex. However, it IS possible to trade them in. If you want a Leafeon, you can find someone with an Eevee (or a Leafeon...) and do a Wi-Fi trade.

Note that the blind trade network won't help you here since you have to have seen a pokémon to search for it. But getting someone's freind code and linking up via the bottom floor of the pokémon centre should work.