If you cannot find your diploma (I can't even tell you where mine is after all these years), then go to your high school. They might have a copy or something similar you could use. Hope this helps.
Call your high school
My diploma was destroyed in a car accident. It was in a box in the trunk and was thrown from the car when rear-ended and landed in a ditch. (Along with all my yearbooks,) I contacted my school and asked for a new one. They told me I couldn't get one, but that my transcripts would work just as well. So I just ordered a copy of my transcripts and used that whenever I needed proof of graduation.