I got a 06 monte carlo Ltz with the 3.9 engine i can not find any performance parts or exuast or anything what is some good site to get performance parts i want more hp

 

There is pretty much nothing out there for the 3.5, and 3.9. Magnaflow makes a catback system for the 3.9 but you'll have to buy the SS kit to fit the cutouts. Also MPD makes CAI that will fit the 3.9. Other than that you got to go custom.
Check out hottexhaust.com for the Magnaflow Catback
Also mpd1.com for the intake.