Maybe try Peter Holmes - he seems to be still making weights containing complex canes to the same design as the stones, and may be able to make a stone, if he doesnt already have some. Also I have seen for sale on ebay Jewellery containing stones that was recently made by ....somebody in Crieff.
However I do recall when one of the Caithness people (was it Margot Thiompson) came to give a talk to the NPS a few years ago she had a bag of stones so Frank's suggestion might be a good one.
However a couple of very rings have just sold on ebay for around £20, (see 160081072032 and 160081071983 ) so don't spend too much on it.