Yeah, it's really unlikely you can do this (whatever the fake reviews the recovery tool distributors post say). The messages are stored in an area you can't directly access without root and which cannot be accessed over USB, and they are not stored as a set of separate files (which is what conventional file recovery is designed to find) but as entries in a database. I suspect a proper forensic recovery service could do something, provided you stop using the phone in the meanwhile, but that will cost serious money.
There are message backup tools that can run regularly on a schedule, or even automatically when you receive a message (SMS Backup+, which backs up your messages to a folder in your GMail account, can do this). So if preserving your messages is important I suggest you look at these. It won't help you now, but might in future.
