The AWS ACM does not provide an API to download the public key of an ACM SSL certificate.
However, once you have your ACM certificate setup on an ELB or CloudFront, the public key will be served when you connect to it via HTTPS. From there, you may be able to save the public key.
Try using OpenSSL to get and save the key:
openssl s_client -connect the.host.name:443 | openssl x509 -pubkey -noout