You should file a Freedom of Information Act (FOIA) request online. You should be able to file and ask for all immigration related documents relevant to your case. They should be able to give you any documents that can prove that your I-140 was approved. Remember, even without the proof you should be entitled to almost all the benefits because the I-140 is the employer's case and you may not have a copy of the approval but you are the beneficiary of that case so the government should cooperate.
If you do not have an issue with extension beyond six years you are fine. Even if they revoke your I-140 one day after your I-140 approval, they cannot take away your priority date. That can be transferred to another new green card. If they don't revoke the I-140 or if they revoke it after 180 days your spouse will be entitled to H-4 EAD. But if they revoke it before the 180 days then both you and your spouse will have to wait until your second I-140 to a new employer is approved. FAQ in detail...
Note: This is a verbatim transcript of the referenced audio/video media delivered as oral communication, and, therefore, may not conform to written grammatical or syntactical form.