Radiant Crystal now can be purchased from the NPC in Morroc, ‘Dapara’ the Mastery Point Exchange cat. (Standing next to Gilgamesh). Radiant crystal sold at the cost of 20 Master point.
Alternatively, you may find them at Auction House. The price on bid are around 500-900 Zeny for one full stack(255units).
Lastly, the conventional method of getting radiant crystal is dissembling level 40-49 Equipments. Which is not so cost effective, because there are chances to fail and receive nothing during dissembling.