I would really suggest ebay if he wanted to just get rid of the parts, I saw a Trinity 2-1CV Complete setup go for over $400 a week ago, and you will for sure get more than 600 for the front elkas.
I'm assuming you have stock carbs (that big of a jet) Cheapest place to get K&N Clamp on's for STOCK carbs is RockyMountainATV. IF you have bigger carbs check out Magic Racing.
Yamaha dealer might not have it, check local small engine shops/stores, I get mine from a place like that 1 Gallon for $30. That's for the Dominator 50:1.