Your Social Insurance Number (SIN) can be found in several places:

1. On your SIN card or the Confirmation of SIN letter issued by Service Canada.
2. On Canada Revenue Agency (CRA) documents like tax returns, tax slips, records of employment, or RRSP contribution documents.
3. In your My Service Canada Account (MSCA) online, where you can view and print your SIN once registered.

If you lost your SIN, you can get a replacement confirmation letter by applying in person at a Service Canada Centre, online, or by mail. To access your SIN in MSCA, you need to register and verify your identity using information such as your birth registration number or Unique Client Identifier (UCI).