See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question.
First of all you really don’t need a copy of the I-140 approval. We have been able to get H-1 extensions as well as priority date transfers based upon just your name, social security number, date of birth , name of the old employer, but you should file a Freedom of Information Act request. If you go to the USCIS list of forms you will be able to see FOIA Freedom of Information Act. Strictly speaking it is a Privacy Act Request but they are the same form. The form is G-639. That will give you a copy of your documentation. Often time’s government gives you a copy of all your paperwork including your I-140 approvals. So doesn’t hurt to try that way. So we don’t really need it. You should be ok. But it is a good idea to have copies of everything that the government has and can share with you. So when you file the FOIA/Privacy Act Request form it’s free to file and there is no charge on it. In a few months (4-5 months) you will get copies of the documents and it could very well include I-140 approval as well.