Just a heads up, if you do find an intake from a large company they are going to screw u cuz' you drive a GS... like $350+ for a piece of mandrel aluminum and a $25 filter. Can you cut pipe and turn a screw driver? then a custom intake isn't really that hard. Made 2 itakes for friends just out of scrap from previous projects.
If you still want to waste some dough try these sites:
Luxurymods.com
L-tunedparts.com
Lextasy.com
Stillen.com
I know SRT makes a performance 4.3L intake but its like $400 but comes w/ an air/fuel chip.