The I-797C IS the copy sent to an employer where an attorney handled the case -what you probably want is the I-797 or I-797A, which is considered the original (and, if a change, extension or amendment of status was requested and granted, has the new I-94 printed onto the bottom).
A duplicate can be obtained by filing form I-824 Application for Action on an Approved Petition - but, what was the I-797 for? An employment-based nonimmigrant visa? Something else? If a petition, the petitioner must normally be the one to file (the sponsoring company). Would need to be the applicant for an application.
Speak with an attorney for further guidance.
I agree with the above answer. You may also use the receipt # on the USCIS's online system so you can show that the case has been approved.
If the case was filed by an employer, the employer should request the duplicate, and it would be sent to the employer.