I dont know avout GA. Here in VA i took a Rinnai tankless certification class it was free to take and was done by rinnai at r.e. michels. I saw somewhere it said you had to be certified to buy them but i dont think theyre really strict on it.
You can buy tankless heaters any where. Make sure you know what you are doing if you install it yourself. It is very easy to set up very dangerous conditions. If you have hard water, you must provide conditioning or void the warranty. They can build up scale very quickly.