Here's a hint applicable to any carrier: keep a used recharge voucher or the original SIM card package with you, so you can explain to the 7-11 or Circle K clerk easily what you want when it's time to buy a recharge card. All the convenience stores carry them but I've never run into anything except blank stares when I try to explain it to the clerk in English. (Even from the ones who speak English reasonably well.) But, show them an example of what you want, and presto, there it is.