The TIN is on the Form 1009-R that you get from DFAS. You have to get the 1099-R. You need more than just the TIN. Don't guess at what information will be on the 1099-R. If you didn't get it, contact DFAS to get a replacement copy, or check the DFAS web site.
As rjs stated you can check if can access your Form 1099-R on the DFAS mypay site.
Please be aware if you meet the TDRL/PDRL Exemption you might not get a 1099-R.
"TDRL/PDRL Exemption: If you retired under a disability law (Temporary Disability Retirement List or Permanent Disability Retirement List), your retired pay will be fully non-taxable if your pay is calculated based upon your military (not VA) disability percentage and you meet one of the following conditions:
The welcome letter you received from DFAS when you first retired indicates whether your pay is computed using your military percentage of disability or your years of service." (DFAS)