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If you live in an area that has a good Japanese supermarket (e.g. Nijiya, Mitsuwa), see if you can find stock powder. The common one of course is bonito which you don't want. See if they have pure mushroom or konbu powder, like the first ones (left most sides) on the first and third row of this shop in Japan:
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I would probably just make a kelp stock, but there are some great suggestions here:





