The SIN is that of the transferree (person share is being transferred to). It's optional, you are not expected to ask for it, and you can leave it blank if you're submitting the completed and guaranteed certificate to the transfer agent.
However, usually with an in-person trade, it's appropriate to sell the completed and guaranteed certificate to the transferree, so that they can either pay the postage or walk it in if there's a local TA office. In that case they can choose to fill in that number if they want. If it's left blank the TA will eventually get it through mailing them paperwork to tax certify the account (or Computershare allows them to tax certify online).