Like they said, you can buy or make one. I personally have a K&N in mine and love it, installation was a breeze with the exception the bolt my wiring clips to the tranny on was stripped so I had to X-thread a smaller nut on :p
I have heard really good things about the Wizaired one also.
You'll notice a decent differrence in the low end too, and it sounds cool when the S/C winds out.
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K&N CAI, custom 3" Dp, U bend delete, resinator removed. high flow cat, 3.5" pulley, incar Overkill PCM tune, 180* tstat.