How do I find my payers federal id number

[Carl]

It's  the form sent to you by the entity that paid you. Where exactly on that form, depends on the form you were sent. Generally, it's in a box labeled something like "Federal Employer Identification Number".

[VolvoGirl]

What form are you trying to enter?  There is no EIN on social security SSA-1099.  If you got a SSA-1099 you are entering it in the wrong place.