I got the AMI using the awscli for the us-east-1 region, but you are right I missed the ENA support parameter.
To summarize, as long as the instance type is C4/C5, the kernel module ixgbevf is loaded (ENA), and AMI virtualization type is hvm, then enhanced networking should be enabled.
To get the latest AMI for Ubuntu trusty 14.04 with ENA enabled on us-east-1 I ended up with this AWS CLI:
aws ec2 describe-images \ --owners 099720109477 \ --filters Name=root-device-type,Values=ebs \ Name=architecture,Values=x86_64 \ Name=name,Values='*hvm-ssd/ubuntu-trusty-14.04*' \ Name=ena-support,Values=true \ --query 'sort_by(Images, &Name)[-1].[ImageId,CreationDate]' \ --output text --region us-east-1 ami-f0f8d695 2017-11-21T15:21:29.000Z
Another useful resource is the Amazon EC2 AMI Locator
How to get the owner id?
Check this article