You want to file jointly, but Ex won't cooperate. No reason to panic. OK, let's try again. Even for this year, it's not too late. Certainly for year 2000, you're early.
Some people even use a shoebox.
Or a file. Or a drawer. But get them all in ONE place. If it has a number on it, put it in. For records your Ex has, assuming no cooperation, try contacting the third party for information.