If you already created an instance using the Marketplace AMI, you can find the AMI ID in the Description of your EC2 instance.
Otherwise, use the AWS Management Console to find the AMI:
Using the AWS Management Console
- Go to the AWS Marketplace page and search for the image.
- Click on the image title.
- Click Continue to Subscribe button.
- Click Continue to Configuration button.
- Verify Fulfillment Option, Software Version, and Region values. Changing any of these can change the Ami Id.
- Find the Ami Id. For example:

Thanks for showing me how to get a Marketplace AMI. I needed to update a CloudFormation template with a new AMI, and they sure make it difficult to figure out. You rock!