Narragansett Arms went out of business many years ago. The Rifle Shoppe made the guns on contract for them and it was a limited run, although the Rifle Shoppe does sell parts that would allow you to construct a Ferguson of your own. The limited edition guns pop up for sale every once in a while, so search online and you may have some luck. $3-4k is is the usual range. If you are interested, there several books out there about the care and feeding of the Ferguson.
I've got one (repro) and it's an interesting gun to shoot. Kinda finicky about the lube you use on the breech screw, but a lot of fun. It's a lot handier than my long rifles because it's shorter and lighter, and just about as accurate. Unlike a long rifle, the Ferguson mounts a very long, flat, sword type socket bayonet.
Hope this helps.