I love scarf rings and use them on my twills all the time. Mostly to compensate for being knot-tying challenged. I love Cosmos, but truthfully haven't used my Cosmos rings yet. I tried to find the large one, someone here indeed bought one and posted about it awhile ago. I asked H to do a search and they told me several months ago that the large Cosmos is entirely sold out. I've never actually seen one, but I don't get to that many boutiques!
The only one that comes to mind for me for the rings for shawls is the one pictured in the thread above. You see them from time to time on eBay as well.
I try to purchase other things for scarf rings and recently found some glass rings in varying sizes - I'm thinking the big size will do for a shawl, though I haven't tried it yet. There is also a size that should work nicely for twill carres, so I bought a bunch of colors! Check a fabric store - where I live we have a very nice local rather high end fabric store and they have fab notions, inclulding all sorts of interesting belt buckles. I've purchased several of them as scarf rings. The only thing you have to be sure of, though, is that there is nothing on the inside of the ring that could snag a scarf so I always run my fingers around on the inside several times before buying. People think I'm nuts, but if they knew I was going to use belt buckles as scarf rings, they'd probably think that anyway!!
Good luck!