Search on google for 'NS10 replacement Tweeter' and a bunch of companies will come up. I got some from Speaker City but they may be out right now. Expect to pay around $65. Not sure about the crossovers.
Call Yamaha's service department. Unfortunatly, I believe they are back ordered indefenatly for tweeters, and they have anounced they are going to stop stocking the woofers as well.
I am afraid it is time for you to get new monitors.