The header choices for the D16A1 are fairly limited. For budget choices, DC Sports and Lightspeed are probably the best choices. Either one can be had for under $300. The big problem is that both of these companies no longer produce, or stock these headers. Undoubtedly, you could get higher gains off a custom manufactured race header, with a lot of R&D, but expect to shell out upwards of $600.
I have heard that the DC Sports is dyno-proven to make the most power for the inexpensive headers, though I am yet to see any dyno charts posted on the matter. This is the header I decided to go with. It is available in either stainless or ceramic coated varieties. I wanted the stainless, but for budget purposes, decided to stick with the ceramic.
So I tried to order the header from 2 companies (one of them being NOPI, assuming they were large enough to get anything). In both cases, I was informed that the header was no longer available. Apparently neither company actually stocked them, and both went back to DC Sports to get it, but of course why would DCS have any??? So what now? One word - eBay! I got a lightly used header from some performance shop that was selling their display model. $290 + shipping, and its the stainless one that I originally wanted. It was definitely a little more than I wanted to spend, but there isn't much of a choice, since these things are getting really hard to find.
Its definitely a nice looking header. The place I got it from said it had never been used, but it looks like it was bolted on to something because you can see where the bolts touched. There is also some grinding done on the ports, and I'm not sure if that's normal or if someone tried to mess with it. Either way, can't be worse than stock!