You can send email to cell phones. Here are the addresses for the top six US carriers. Substitute an actual phone number, send the e-mail and it will arrive as a text message.
Virgin Mobile email@example.com
Or you don't know the cellular service company, go through a third-party service, rather than direct e-mail, consider using http://www.teleflip.com. Teleflip automatically sends e-mail to the proper service provider when you enter the 10-digit phone number.