Hello, I have been researching your issue further. I found this link that indicates Sprint does NOT save text data, only the phone number and time stamp, and only for up to two weeks. Sounds like what they told you was correct. http://www.ehow.com/facts_7345734_can-messages-retrieved-after-deleted_.html
The only texts you will be able to retrieve are those that you can manually find like you have been doing. So it appears that the most recent twenty are what you have access to.