I don't know of any stock/App Store apps. However, if you are jail broken, you can get the message tweak biteSMS. It has the ability to schedule texts to send later.
Does this app go through my provider so txt messages won't cost anything if got free ones?? Noticed in setting that u can change who u wanna sent it through? Also changed my message alerts to come through that not my normal one??
The biteSMS tweak's scheduling function only works on your carrier's texting plan (it won't work with iMessage). So, if your text plan is unlimited texts, it'll be fine.