I would really like to get this for my girlfriend though I unfortunately do not have any information about it! I believe that it is Tiffany&Co. though I am not 100% sure. If you have any information about the necklace it would be much appreciated! Thanks!
You might give this site a try: http://initialelegance.com/initial-necklaces Note - don't know anything about this company or their quality; I found them on an internet search and they seem to have something similar to what you're looking for.
To do your own search try googling "circle initial necklace," then click on images, and check out the images that are closest to what you have in mind. In most cases there will be a web link.