Everywhere I've looked, websites just state to buy a ticket when in Cancun, but where can I do this? Are there offices in Cancun or do I have to go to Cancun airport and go directly to Aeromexico or Cubana desk?
Use cubajet.com to buy tickets online. This service supports payment by Visa and MasterCard. Also you may use Maestro card.
As alternative, you always may buy tickets directly in airport. There are official desks of Cubana and Airmexico airlines inside International Airport of Cancun.
But remember to buy ticket at least 4-5 days before the flight. When you buy ticket on the same date - it costs much more.