Three brief ideas:
--The paper has several references to growth methods and the undoped is always the base material.
--Is there a reason why MgSiO3 is not a consideration? I found a large matrix for HgS that turned out to be very clean natural MgSiO3 once, easy to find again, in single crystals about 8mm long.
--John Hanchar at MUN used to quote prices for bulk syntheses, usually fluxed mineral structures. Ask for bulk pricing, it will save you the $$$ of cutting boules.
PS I think CMTaylor's idea of providing 2mm x 2mm x 2mm cubes gave the best price for materials. Neither a gram, nor flyspecks, but the size in between.