Where can I find a copy of my Article 15 (Field Grade) if not in OMPF?

At your rank, an Article 15 should have stayed in the local file for two years or until you PCS, then destroyed. I would still tell them about it just as a matter of integrity and if they catch you lying, the world tends to fall on you.

Art 15 papers are supposed to drop out after a certain period but the question asked if you ever received one for a TS clearance. If you answer no you are lying on a federal form. answer and explain in remarks. It is like have you ever been charged with a felony that you were never taken to court charges were dropped but to answerer the question yes you were charged. They will find out and that will cause you problems for sure.