You can get a lot of ninja toys from martial arts supply houses, I'd recommend the cheaper stuff to practice with, which will also teach you how to evaluate for quality when buying nicer things later. Simple stuff like tonfa I'd just make, it lets you customize the length, weight, and style, plus it's cool to have something that you can't buy in a store. I would also recommend caution when it comes to any segmented or flexible weapon, especially the longer ones such as manriki kusari, kusari gama, Chinese rope dart or meteor hammer, etc. To me, these are some of the few "ninja" things that might be practical in the modern world, but are also the easiest to hurt yourself with if you don't know what you're doing.
As a note on practicality, if you're really looking for a useful, modern martial art, Krav Maga or one of the Kali offshoots are going to serve you far better than anything Japanese, even if they're not authentically "ninja". I favor panatukan myself, it's one of the dirtiest and most effective styles around, especially if you habitually carry a knife or two.
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