AFAIK it cannot be bought or registered anymore. The website for Nordic Global Inc. disappeared from the web five years ago.
http://web.archive.org/web/200503211...com/index.html
As have all other registration websites for Miami
http://web.archive.org/web/200503141....com/mireg.txt
While the software is not free and is still under copyright I am pretty sure you can use it without feeling guilty.
The alternative is AmiTCP from Aminet.