( Are  they  only  acquired by  paying  for  them  with   real  currency?) yes you get 1 free one and thats it

Don't bother  with  em.  The  mechs  usually  become  available in  other  means  later  (just  muuuuch  weaker)  The  event  tokens  is  like  paying  100  bucks  to  be  the  first  to  play  at  a  high  star  rating.  They  usually  will  go  on  sale  for  like  6  bucks  in  a  couple  months.