How can I learn ninjitsu?
The best way to find a dojo in your area that teaches ninjitsu would be to ask around any local martial arts clubs, etc. The yellow pages are still possibly the best way to find a good instructor. Though if you live in a rural area, you might have to travel a great distance. A friend of mine had to travel about 45 minutes one way to see his instructor when he lived in a smaller town of about 40,000 people.
Though if you can't find a ninjitsu instructor, you might consider trying jujitsu, which has some similar grapples & traps, as those are typically more useful to learn than karate, which is the most popular japanese martial art.
The best way to learn ninjitsu is on your friends in forests during fall. Sneak around. Ninjitsu is literally the art of being sneaky. Theres nothing else to it. However if you want to be a NINJA, you'll probably need to go through several different styles of martial arts until you find one that's good for you. But be warned. Teng tsu do is absolutely POINTLESS since it's only basics on HOW to punch and HOW to kick... then you're done. waste of money.