Emotional Support Animals
Sadly/Conviently?, there is no real governmental governing body to govern if animals can be considered emotional support animals. Therefore, you can get it signed off by a therapist or a doctor. There are some online services that will create you a remote meeting for $100-$200 with a licensed therepaist to evaluate if you are ESA viable.
Service Dog
No registeration required, nobody can ask for proof... But you can do a certification process.
Self certification sites online, but you can pay a program for $15k+.