When you say music theory...what exactly do you mean? General music theory is like learning to read sheet music or learning the scales or relative chords within a scale. If that's your meaning then there's not really anything specific to the instrument, course if you're after playing technique (like for guitar, drop your thumb to the middle of the neck or loosly clench your fist when you play). Or are you after a some notation? If you're going for accurate you're better off buying books, if that's out of the question, try:www.olga.netwww.guitartabs.ccwww.ultimate-guitar.com
Not sure about any of the other instruments, google is your friend though (unless you object to their censoring in China).