Where to find organization ID for Portfolio tier

If you are referring to the organization ID of AWS Organizations, you can check it from the Organizations screen even if you are a member account.
https://docs.aws.amazon.com/organizations/latest/userguide/orgs_manage_org_details.html

Hi, it is a number that should be provided by the organization sponsoring your credits (e.g. venture capital investor, accelerator, incubator, etc.). It identifies them as an approved "portfolio (of companies) owner", ask who manages relationships with AWS for them

https://aws.amazon.com/activate/faq/