Are you referring to the Payer's Federal ID number; i.e., the institution that sent you the form?
If
your 1099-R form is the standard IRS version and not a "Substitute"
version, it should be on the left side just below the Payer's name and
address.
It may say "Payer's TIN". It should be just to the left of
your TIN (the recipient's TIN.)