Aprillen: The only problem with silver disks is that the material's expensive. I'm sure though that they could be easily made in large amounts by f.ex. etching the image on, if using photo etching you could even create lots of near identical ones. Etching as a process always varies a little from plate to plate no matter what but with photo etching the initial design would at least always be the same. Hmmm... Then again some cheaper metal could also be used and just silverplated afterwards...
For mother of pearl, there's at least one abalone farm here because the north coast waters are apparently ideal for red abalone. There are a few other mollusks too that produce a mother-of-pearl layer but they tend to be on the small side.
Yes, but the good thing about silver is that you can melt it down and reuse it. So if you found a lot of pretty but ultimately not very useful silverware, for instance, it would be pretty easy to make new things from it. Silver does corrode with time but most of it is still intact. Gold and silver stay around, if they've been around to begin with, and since they aren't very useful for tools and other practical things, they might as well be reused for pretty things. Mother-of-pearl can't be worked like that -- you'd have to find new shells of the right size, and right now (Y90) diving for nacre shells in sea-beast-infected seas doesn't seem like a good use of anyone's time. Also, silver has long been used for ornamentation on traditional folklore clothing in Scandinavia, especially in Norway, southern Sweden and Sápmi (and I wouldn't be surprised if this goes for parts of Finland too). In a culture where you can't show off your wealth with new cars or the latest electronic gadgets, good clothing and precious metals are your fall-back method for showing off.
Also, silver is mythologically connected with the Moon, and metal decorations have been prevalent in shaman clothing, so silver discs appearing on a Spirit/Dream World outfit makes perfect sense to me.
Tl;dr Mother-of-pearl is not out of the question, but silver seems like a much more likely candidate in my mind.
EDITED TO ADD: The above reasoning is for the presence of silver on Reynir's Dream World outfit
in the comic, not necessarily for the cosplay outfits we make ourselves! Just thought I'd clarify that.
