Sorry to hear about that Ryan. I hope working on the bike keeps your Dad close to you.
Try:
http://www.vintagebikeads.comhttp://www.vintagmx.ushttp://www.bikeboneyard.comAlso, try Google like this: Put the model of your bike in the search field. Then also type craigslist.org, then hit search. You will be surprised at how much you can find there.
There's a big Yamaha shop that buys old parts from other shops. I think they are called Speed and Sport Yamaha. Is that right? They have tons of old YZ stuff.
Enjoy!
Jon