Using Google API for Python- where do I get the client_secrets.json file from?

Go to credentials tab. Client IDs will be listed there. In the right, there is an option to download it (.json file)

If you go to your Google developers console you should see a section titled OAuth 2.0 client IDs. Click on an entry in that list, and you will see a number of fields, including Client secret.

If you have not yet created credentials, click the Create credentials button, and follow the instructions to create new credentials, and then follow the steps outlined above to find the Client secret.