You can buy them from the store, but otherwise, just farming and leveling your 1 and 2-star champs from  gems.

Good  morning,

The  easiest  way  to  get  chickens  is  to  buy  them.  You  can  get  random  chickens  here  and  there  by  completing  content  in  the  game  but  they  aren't  always  5  star.  

However,  the  more  time  consuming  way,  and  the  way  most  players  do  it,  is  to  level  up  "food"  champions.  Which  just  means  to  level  champions  you  don't  want  until  they  are  also  five  stars  so  you  can  feed  them  to  Skullcrown.