I honestly doubt James will post the place he gets them from, due to the possibility of people just buying them up and flooding the market.
They are however not cheap unless bought in bulk I believe, so you may be happier using the glass decorative gems suggested above. You could also just search online - you're bound to find plastic gems.
Defend the Land for it is the Base of the Pillar,
Elawyn Featherthorn Brightstar
Acolyte of the Circle of Ten & Chosen of Fey Lord Tillion
Guardian of Belladeen
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OOG: Kristen M.