You can get a knife from a Circle K, but definitely from any airsoft shop. You'd be surprised that you will be able to find one relatively easily. I do believe that pepper spray is illegal there. It's a pretty safe bet that you would'nt be murdered in Japan as the hard criminals tend to stay away from foreigners. your biggest "threat" would be late night drunks or maybe a small gang of young punks...still not worth it spending time in Japanese police holding. 60 or 120 lumens in the eyes works really well and enough to get yourself out of any minor trouble. The Japanese culture is really 2-sided and it is pretty invisible to outsiders unless someone "helps" bring it to light. The Japanese & Koreans really don't get along very well and the Jap gov't takes care of the homeless by tossing them in jail or running them out of the city limits. Even the permanent residents are treated to the "papers please" and other indignities and worse yet the naturalized citizens are still treated like foreign garbage. A friend
www.debito.org has a really eye opening site not only about his experiences but other perm residents & ex-pats.