Missing Module

Hello there,
I was told that we can upgrade an era units for free up to era lvl 65 witch I'm not able to. I miss a lot of MK V Cresh Ability Module to upgrade my unit. Should I buy it for crystals or it should be as a reward from coliseum but I did not obtain it. Maybe be there is another way or I missed something? Please explain if you can. 

Thank you all and may the force be with you