Deleted messages can only be obtained from Sprint via court order and may only be given by law enforcement officials. After 14 days, all messages are permanently deleted by Sprint and are cannot be obtained by anyone.
If you mean get back a deleted message, i don't believe you can, as i have tried before and my efforts were fruitless. I'm pretty sure the only way to get that kind of information would be if a police station or court provided a warrant for old files as evidence in a case.
There are several types of messages from Sprint including SMS and E-Mail. SMS is sent directly to your device and no additional record is maintained so if deleted on device, its gone. If you have a Sprint mobile e-mail address and using the web based solution, there is an option to view deleted messages folder.