well i dont know where thottbot says that they are found from but i've gotten some from the naga in zangarmarsh. basically the clams that they drop can give jaggal pearl or shadow pearl. i also confirmed this with allakhazam's item search.
as for JC after 350, I know some people recommend to buy a gem cut recipe and try to level up on that, cutting the gem to sell to make some money back, or for people who need it. these recipes are roughly 200-300g or more on my server also.
i went the route of buying some more ring and necklace recipes, costing about 200g total for 3 recipes. they take more mats to make than just a blue gem, but they helped me reach 300 with no problems other than collecting a lot of adamantite ore and primal earth, which you would do anyway for your JC mats.