It depends on what you want and how much you want to afford. There are a couple of options out there. There is the Volant, the AFE or the K&N. Check this site out for prices. They are pretty reasonable.
I've seen the same with the Spec V market... People telling me that I am a sucker paying 500 out the door for one header and they got a 125 dollar ebay stainless one that is just as good.
Year later most were complaining that the flanges were breaking and not one broken Nismo header 3+ years later.
Same thing with aftermarket CAI's. Fit and finish first rate with Nismo, not so with some of the other manufactures.