Social Security Questions? Ask a Social Security Expert
One way to get a to get her Social Security is through your income tax return filed when you were married. Her Social Security number would have to be on either a joint return or a married filing separate return. However, I am not sure if you ever filed as married filing separately, and if you did, whether you had enough income to file a tax return. Nevertheless, you can request a copy of your prior year tax return from the IRS by visiting this link: http://goo.gl/GLGj5P and completing Form 4506. You can also request request a copy of the return transcript which should also have her Social Security number on it by visiting this link: http://goo.gl/2HRL2j and completing Form 4506-T. In addition, if you hired a paid tax return preparer he or she might still have copies of your old tax returns, depending on how long ago they were filed. Only one spouse is required to sign either of these returns even if a joint return was filed.
I hope this answers your question. Let me know if I can clarify anything or answer any additional questions. Jonathan
You actually may not need you former wife's Social Security number when you apply for spousal benefits (the Social Security Administration may be able to find it). See this website: http://goo.gl/DlmVK4.
Let me know if you have tried applying without her Social Security number and the SSA's response.