Here is one possible solution: if your last saved iOS software backup via iTunes contained the messages you want, you can apply it and then view the messages.
One important setting to change in iTunes before you attach your iPhone is to turn OFF automatic syncing because the instant you plug in your iPhone iTunes will automatically update your backup and our previous backup is gone. This can be performed by opening iTunes -> Edit menu -> Preferences -> Devices tab -> check mark "Prevent iPods, iPhones, and iPads from syncing automatically". Now plug in your iPhone, select your device from the left-hand column and choose "Restore Backup..." from the Summary page. Follow the on-screen prompts.
Note: Anything you installed, changed, etc on your iPhone since that backup will not appear on your iPhone after we apply the backup. BUT you will get to view your deleted messages.