You can buy the ticket for internet or go to a travel agency. But if you have use of the networks you can get on the network than any airline and buy the tickets right there but you would need a credit card. If you go to the airport you can also buy it.
Sorry am not a Spanish speaker so I had to translate my response in google translate...here is the response. You can buy the air tickets through any airline websites like American Airlines or united or third party websites like kayak or even you can search air tickets through google/flight
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