Share 'Using a GSM modem in the U.S.'

Sorry if this is a repeated question, but I'm having a difficult time identifying a GSM modem and a service provider just for sending texts in the U.S. I went into a T-mobile store yesterday hoping they'd know what I was talking about and got blank stares back, they said they only have data sticks and was confused about why I would want a modem to send text messages when I could do this using a p…