You can get a telcel SIM card for around $10 everywhere, even in convenience stores. You can also add airtime by phone with a credit/debit card.
Another option is to buy a burner phone, Nokias 100 are available for around $20 and the battery should last more than 10 days, but you wont get nothing on it but calls and messages.
Another option is to buy a burner phone, Nokias 100 are available for around $20 and the battery should last more than 10 days, but you wont get nothing on it but calls and messages.