How do i get a social security number from a business associate

The information is required on your tax return so you can mail your return with a statement and contact information for the buyer, including address and telephone number.

If you explain this to the buyer they may be more likely to be cooperative. You can have the buyer complete a IRS Form W9 so that you can maintain it in your records and it will look official.  It's simply a document for anyone you might need to issue a Form 1099-INT (or other 1099 income reporting document) each year.

Information here can be obtained for you to show the buyer so they can read for themselves the reporting requirement.

Seller-financed mortgage.    If you finance the sale of your home to an individual, both you and the buyer may have to follow special reporting procedures.

1. When you report interest income received from a buyer who uses the property as a personal residence, write the buyer's name, address, and social security number (SSN) on line 1 of Schedule B (Form 1040A or 1040).
2. When deducting the mortgage interest, the buyer must write your name, address, and SSN on line 11 of Schedule A (Form 1040).
3. If either person fails to include the other person's SSN, a penalty will be assessed. This last item should spark some cooperation.
   

Publication 537-Installment Sales - for more information you can use this hyperlink.