True about not using Persions unless you have some around, but then, we should all have a box or two of the Persions hanging around - it's a cool little set with a bunch of figures that can be mixed into all sorts of crowds! :D But, I wouldn't buy a box specifically for ninjas, either.
Lauging at the "anyone can be a ninja" thing, and it's true!
The story (as I hear it) is that in the real world, there wasn't a real ninja costume, they really did just dress like anyone else, or dressed the part of servants like gardeners or cooks or manual laborers or whatever that would be so far beneath important people that their targets wouldn't notice them - the modern equivalent would be that they'd dress like janitors or just faces in a crowd or whatever - "grey men".
The ninja costume as we know it is supposedly an artifact of opera and theatre, as the costumes worn by stage-hands so that they would blend into the background - they were such a routine part of the theatre experience, audiences had learned to tune these stagehands out and pay attention to the play's characters. Some genius playwrite or another needed ninja assassins to spring out of nowhere to murder an important character, and decided that, rather than doing something obvious like make the assassins spear-carrier or servant characters in the play, the assassins would instead spring out of the shadows dressed as the stage hands that the audiences had learned to ignore, and thus the ninja costume was born!
One can only imagine the surprise of the audience when the ninjas turned out to be those lowly stagehands that nobody pays attention to. ANYONE CAN BE A NINJA! :D
So, I guess the stereotypical ninja costume is more fantasy than history, not that there's anything wrong with that! The more "accurate" costumes would be something that would look like the costumes of common men of their historical era, and anything that looks "close enough" to the period costumees of Japanese peggars, farmers, servants, bureaucrats, messengers, laborers, and the like ought to work. (But, they maybe wouldn't be quite as cool as the fantasy ninja costume we all know and love!)