Loose front sprocket nut, ouch!

I was doing some seasonal maintenance to my bike and found a mess with my front sprocket. The nut had come loose a while ago and the splines on the shaft and sprocket got buggered up. I should have peened the nut down when I properly torqued my new sprocket on but never saw a nut like that before, and didn't think about it. Never noticed it while riding. I already know I need a new sprocket but hope I don't need a new shaft. I hope to slide a new front sprocket on there and torque it down to spec then peen that stupid nut and keep on riding. Help to assure me that is all I need. I am well aware that I screwed up and I should have checked it more frequently.

I've read from more than one rider on here claiming they weren't gonna' stake the nut down on purpose because it was a hassle to get the nut back off.
Imagine that?
I can tell you that I always used a new nut, I torqued mine down, and then staked both sides of the nut.
I rarely use a torque wrench, but this is usually one of the jobs I use one on (to ensure I get it properly tightened).

If I were in your shoes, I'd apply a thin film of some kind of grease that contains a lot of moly (like moly grease or Bel-Ray anti-seize) to the splines, slip on a NEW sprocket, torque a NEW nut back on, and stake down both sides of the nut.
I think you'll be OK as long as you don't have a repeat performance. :)

I didn't know I had to peen the nut down, I thought it was a regular nut. Lesson learned. I am hoping a new properly fastened sprocket is all thats required to get me going again without worry of the shaft stripping down to bits.

I know you didn't know about the peening part.
There have been others that consciously skipped that part, though, and that blows my mind.

Where can I find a new nut?

Any place that sells Yamaha motorcycle parts.
I wouldn't expect most dealers to stock it, but ordering doesn't take more than a few days, unless the place is run by slackers.

You want the real McCoy that belongs on there.

You can always do what the drz guys do, this is a common issue on that bike...
Lock tight the sprocket to the splines on the shaft and also lock tight the nut, use blue ( medium strenght ) lock tight plus peen the nut too, it wont go anywheres then.