Other than Amazon Certificate Manager as moatra mentions (which I don't think let's you export the certificate), I don't think there is currently an option for free wildcard certificates.
As an alternative you could incorporate provisioning of a Let's Encrypt certificate for the new subdomain into your deployment process since the process is designed to be automated.
As an alternative you could incorporate provisioning of a Let's Encrypt certificate for the new subdomain into your deployment process since the process is designed to be automated.