You could only get them if you sued and they were relevant to the suit. Your personnel file and everything in it, including written warnings, are about you, but they are not your property--they belong to the company. The company is under no general obligation to share them with you (you may, of course, ask). If there is a lawsuit and the warnings would be relevant to the suit, then there is a legal process (called "discovery") which would enable you to get the documents.