The easiest way is to get them through ticketmaster:
https://www.ticketmaster.com.mx/
However, both nationals and foreigners often have problems to buy them, since it seems that everything is fine and at the moment of making the payment it never approves it, we recommend you to try several times or with different cards, since even with Mexican debit or credit cards it doesn’t work very well either, since their system is not very effective.