There is a neat little weapon called a Kubaton that is a keychain and can inflict more pain than I would ever want to feel! My son's step-brother owns a couple of TaeKwon-Do schools and he also teaches some self-defense classes. This weapon is one of the things that he uses in teaching you to defend yourself. This is the website that sells them and lots of other "objects of pain"!
You would absolutely need instruction but I'm sure there would be lots of places where you could find it.
http://www.selfdefenseproducts.com/kub.htm