you can contact the folks who administer the exams and get your scores, but I think it will cost you a fee. (I was shocked when I actually found my original score report form from... well... um... way longer ago than that. Let's say my SAT scores are older than some of my current med school classmates.)
I contacted my High School and I was told by them that they no longer had the records (1985). They offered no help other than that so that is what I listed when I did mine.
Not a problem as far as I'm aware. Skip it (I did) and annotate reason (i.e. non-traditional, more than 10 years ago, scores no longer available) if an explanation field is provided. Cheers.
(As noted above, this must be a problem for a lot of nontrads. I'm OCDish enough about numbers that I remember my two main scores from ~15 years ago - 740V, 720M pre recentering - but I suspect most people this far out have a life and even being as big a dork as I am, I have no idea what my TSWE score was.)