How do I get proof of graduation if my college closed?

The state-level department of higher education in the state where your college was located will often have access to transcripts and other records from closed colleges, even often private colleges.  If not, they can likely direct you to what you need.  There are procedures to preserve such records when schools close, so they shouldn't be lost forever.

You are likely out of luck. You will most likely need to redo college.