I am not anything close to expert on these flutes. I've dabbled with them at times though. I know enough to be dangerous and probably misguided. But... some thoughts.
You might want to talk to a couple of makers. Since you are a low whistle player you might start with a couple of NAF makers that also make low whistles. They'll have the background to help you with comparisons I would think. Does that make any sense?
Geoffrey Ellis is the first one I would look to. His flutes and whistles are very well done. He's been making Native style flutes longer than he has Irish flutes and whistles. You can get his contact info from his website.
http://www.ellisflutes.com/Another whistle maker that I believe makes NAF is Ronaldo Reyburn. I think he recently offered a couple of flutes on the World/Folk Winds forum. His website is here.
http://reyburnwhistles.com/A number of members on this forum and on the Flute forum have mentioned playing NAF in the past. Maybe they will stop by with their suggestions too.
Hope that helps.
Feadoggie