Daiken is correct, you must NOT learn from a video or book, for one, they can only teach you so much, secondly they can't help you with questions and the like, and they can't help you understand something if you have mis-interpreted it like a proper instructor can. Anyone who offers to teach you moves and the like over the net is a fraud, or insane, or just a child. (who wouldnt have been allowed to learn it anyways!) It is more acceptable if they try and teach you the philosophy behind it, but anything else is utter pants, try going to seminars, there are bound to be some in your local area.
If you can't find a ninjustu club in your area (of any sect) i suggest finding an aikido joint, to my eyes it look like the closest thing, the only alternative thing that can teach you good taisabaki, however that is not to say they are similair or the same, I will have to join a club for my verdict on that, but Im pretty sure they are very different in some respects.
The different sects of ninjustsu concern themesleves with different aspects of the art. If you think sword play and ancient weapons is a load of tosh, then you might want to join a bbd dojo (a modern style ninjutsu organisation). If you want the complete ball game, try bujinkan, genbukan or jinenkan, i personally think bujinkan is getting a bit comercial (to put it lightly) so i suggest learning from all denominations eventually. What you must do is to get a DVD of the old grandmaster before any of these splinter groups came about, his name is 'Takamstsu', also known by the old taoist sages/chinese gung fu masters as 'the mongolian tiger', and he was the last real ninja in my opionion. I suggest talking to someone who already does it to see if its your cup of tea.
ps. Anyone thinking of taking up aikido would best have a chat with DarkDuck, as he is the resident dude around here!
(I myself will get round to just that!) Kendamu, why don't you make this thread into a internal/external martial arts thread, since ninjustu and aikido are the only two martial arts (as far as i know) that fit into this category. Then peeps could discuss them here!