amazon ec2 assign domain name
That's correct. Amazon does not provide DNS service so you need to use a third-party provider.
I don't have enough experience to recommend a particular provider, but I have heard of others using DynDNS, and it appears to support SRV records:
http://www.dyndns.com/support/kb/record_types_supported_in_custom_dns_expert_interface.html#srv_record