I was wondering if it is possible to obtain fingerprints of an envelope that is fairly old and weathered, and if th accused is not the last person to handle the envelope??

YES it is possible.
Request the help of the Police.

Its not necessary to ask the police about this. There are Private detectives and public forensic labs that will do this for a fee.

In fact, unless there is an issue of a crime being prosecuted, the police are unlikely to help. Their crime labs are already overloaded.