Clover makes two styles of these metal adjustable ring thimbles.
Choose the one that sits on the flat area between your knuckles or the one with the concave plate which sits at the base of your finger. Customers who sashiko stitch seem to prefer the one with the plate, which makes sense since it is used in the same way as the leather sashiko thimble. (You can find the leather sashiko thimble in the sashiko supplies on this website)
Open since 2005, but currently online only, A Threaded Needle is located in British Columbia, Canada. I am always happy to hear from you by e-mail firstname.lastname@example.org