You'd have better luck posting this in the Yamaha section.
As far as exhaust, if you want something obnoxious, you'd probably have to make something custom. I believe guys are running track-only slip-ons for YZ450F dirt bikes.
As far as ready-made exhaust systems for Phazer Mountain Lites, I personally would look to companies that specialize in 4-stroke sleds. One company is
http://www.hartmaninc.com/ . The systems you can buy out of big catalogs, like Dennis Kirk, are more than likely going to perform and sound alike.
That's about as much as I know. I'm still running the stock exhaust, and probably won't be changing it out anytime soon, so I don't really read up on this topic much.