Evokers can be found in Woodland Mansions, which are a rare structure that spawn in Deep Forest biomes, as well as sometimes appearing during Pillager Raids, which you can find a guide on how to trigger here:
Solution for We're being attacked! In Minecraft: PlayStation 4 EditionAll you need to do is kill an Evoker. The mob is quite dangerous, as it uses magic to summon various attacks which deal quite a bit of damage, so make sure you are geared up with at least a full set of iron armour. When killed, it will also drop a Totem of Undying, which can be very useful.