Any good smith can convert an NEF/H&R barrel to the caliber of your choice within design limits, but there are no aftermarket barrel makers that provide NEF/H&R barrels. There was one, but they are no longer in business.
Wayne York of Oregunsmithing has rechambered/rebored several barrels for members here, he does a good job and they shoot excellent, but he needs an existing barrel to do the job. He's done a .338-06 A-Square, .405 Winchester and .35 Remington for me.
http://home.earthlink.net/%7Eoregunsmithing/index.htmlSome smiths will use a barrel stub, the underlug and chamber section of an NEF/H&R barrel, then thread it and add an aftermarket barrel.
http://www.precisionrifleworks.net/Nef.htmlTim