There are no texting services which will connect you with someone for free; all the texting numbers we turned up charged either on a per text basis, or offered bundled groups of texts at some discounted price.
You might try one of the many free social networking websites on which you can search, or post "personals," for others interested in similar activities.
We found one on LiveJournal called TextMe where one can post messages looking for someone, or read posts from others looking for texting partners (Note: not all people posting there are looking for dirty talk, some just want to chat, so read anyone's personal ad all the way through before texting something that may be offensive).
In late September 2012, a swimming coach in the UK accidentally sent a steamy message intended for his lover to his entire contact list! Since his contact list included numbers for two students he was arrested and charged with "inciting minors to engage in sex." Remember to text carefully!