Are you talking OEM? I'm not even sure you can get just a shock shaft for that. I could be wrong, but I think a Yamaha dealer can only get you a complete shock - or a sub-assembly (p.rod and clevis) at best.
You're probably better off having your's re-chromed.
I'd bet that a replacement OEM shock shaft would cost nearly as much as a whole,better aftermarket shock. Just save yourself the pain of installing a new shaft and saving a 100 $ and just get a whole new shock which will have better action anyways.
Elka suspension in Canada ( www.elkasuspension.com ) can make you a completelt new shock shaft with rebound needle and nut for about $100.00 U.S. dollars. If the shaft is not bent, you can have it replated by Race Tech ( www.race-tech.com )for about $90.00.