The ring, if anyone is still interested, is available online at ebay, some english sites, and some japanese sites. It is called an armour ring and is designed by Vivienne Westwood, an English woman who designed the "punk fashion" clothes that the sex pistols were known for wearing and was married to their manager.(you guys probably know this and are groaning in boredom, sorry) It is pricy, unless you get a copy. Also, I doubt any of the sites sell Nana's exact ring, since Hachi notes that it is an original, made before Vivienne was all the rage. I don't know if Vivienne's styles are really popular in Japan, but if Hachi is representative of youth culture, apparently westwood, the sex pistols, and sid and Nancy are common knowledge. Here is one site I found that's in English:
http://www.hervia.com/shopload.htmlAs for the ring's legality, firstly if it is illegal and you buy it, I doubt anyone is going to care, but I think the ring that the law seemed to be describing is a Knuckle Duster, which Vivienne Westwood also designs, but Nana's is an Armour ring, which I don't think would be classified the same way. If you wanted to, you could say that law deems any ring a hazard.